diff --git a/backend/app/services/sf_express_settlement.py b/backend/app/services/sf_express_settlement.py
new file mode 100644
index 00000000..a5997b8c
--- /dev/null
+++ b/backend/app/services/sf_express_settlement.py
@@ -0,0 +1,133 @@
+"""顺丰月结抬头与结算规则。不访问数据库。"""
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import datetime
+from decimal import Decimal, InvalidOperation, ROUND_HALF_UP
+from typing import Any, Optional
+
+SF_TRANSPORT_MODE = "sfkd"
+SETTLE_UNSETTLED = "wjs"
+SETTLE_SETTLED = "yjs"
+_AMOUNT_QUANT = Decimal("0.01")
+_MAX_AMOUNT = Decimal("9999999999999999.99")
+
+
+@dataclass(frozen=True)
+class FreightHeaderPlan:
+    transport_mode: str
+    waybill_no: Optional[str]
+    freight_amount: Optional[Decimal]
+    settle_status: Optional[str]
+    clear_settle_time: bool
+
+
+def normalize_waybill(value: Any) -> str:
+    return str(value or "").strip()
+
+
+def _is_blank(value: Any) -> bool:
+    return value is None or (isinstance(value, str) and value.strip() == "")
+
+
+def parse_optional_amount(value: Any) -> Optional[Decimal]:
+    if _is_blank(value):
+        return None
+    try:
+        amount = Decimal(str(value))
+    except (InvalidOperation, ValueError):
+        raise ValueError("运费金额格式不正确")
+    if not amount.is_finite():
+        raise ValueError("运费金额格式不正确")
+    if amount < 0:
+        raise ValueError("运费不能为负数")
+    if amount > _MAX_AMOUNT:
+        raise ValueError("运费金额超出范围")
+    return amount.quantize(_AMOUNT_QUANT, rounding=ROUND_HALF_UP)
+
+
+def freight_amount_label(transport_mode: Optional[str], amount: Optional[Decimal]) -> str:
+    if (transport_mode or "") == SF_TRANSPORT_MODE and amount is None:
+        return "月结"
+    if amount is None:
+        return ""
+    return format(Decimal(amount).quantize(_AMOUNT_QUANT, rounding=ROUND_HALF_UP), "f")
+
+
+def plan_freight_header(
+    *,
+    transport_mode: str,
+    waybill_no: Any = None,
+    freight_amount: Any = None,
+) -> FreightHeaderPlan:
+    mode = str(transport_mode or "").strip()
+    if not mode:
+        raise ValueError("请选择运输方式")
+    if mode == SF_TRANSPORT_MODE:
+        waybill = normalize_waybill(waybill_no)
+        if not waybill:
+            raise ValueError("请填写顺丰单号")
+        if len(waybill) > 64:
+            raise ValueError("顺丰单号不能超过 64 个字符")
+        return FreightHeaderPlan(mode, waybill, None, SETTLE_UNSETTLED, True)
+    if normalize_waybill(waybill_no):
+        raise ValueError("该运输方式不能填写顺丰单号")
+    amount = parse_optional_amount(freight_amount)
+    if amount is None:
+        raise ValueError("请填写运费")
+    return FreightHeaderPlan(mode, None, amount, None, True)
+
+
+def resolve_header_for_save(
+    *,
+    transport_mode: str,
+    existing_waybill: Any,
+    existing_amount: Any,
+    existing_settle_status: Any,
+    waybill_supplied: bool,
+    waybill_no: Any,
+    amount_supplied: bool,
+    freight_amount: Any,
+) -> FreightHeaderPlan:
+    if (existing_settle_status or "") == SETTLE_SETTLED:
+        raise ValueError("已结算的运费不能修改运输方式或顺丰单号")
+    if transport_mode == SF_TRANSPORT_MODE:
+        waybill = waybill_no if waybill_supplied else existing_waybill
+        return plan_freight_header(
+            transport_mode=transport_mode, waybill_no=waybill, freight_amount=None
+        )
+    amount = freight_amount if amount_supplied else existing_amount
+    return plan_freight_header(
+        transport_mode=transport_mode, waybill_no="", freight_amount=amount
+    )
+
+
+def plan_settle(*, current_amount: Optional[Decimal], submitted_amount: Any, now: datetime):
+    amount = current_amount if _is_blank(submitted_amount) else parse_optional_amount(submitted_amount)
+    return amount, SETTLE_SETTLED, now
+
+
+def assert_settle_request(ids: list, submitted_amount: Any) -> None:
+    if not ids:
+        raise ValueError("请选择运费批次")
+    if len(ids) != 1 and not _is_blank(submitted_amount):
+        raise ValueError("批量结算不能填写运费")
+
+
+def plan_fill_amount(*, current_amount: Optional[Decimal], settle_status: Optional[str], submitted_amount: Any) -> Decimal:
+    if settle_status != SETTLE_SETTLED or current_amount is not None:
+        raise ValueError("只有已结算且仍为月结的运费可以补填金额")
+    amount = parse_optional_amount(submitted_amount)
+    if amount is None:
+        raise ValueError("请填写运费")
+    return amount
+
+
+def assert_freight_mutable(settle_status: Optional[str], action: str) -> None:
+    if (settle_status or "") != SETTLE_SETTLED:
+        return
+    if action == "delete":
+        raise ValueError("已结算的运费不能删除")
+    if action == "items":
+        raise ValueError("已结算的运费不能调整样本")
+    raise ValueError("已结算的运费不能修改运输方式或顺丰单号")
diff --git a/backend/tests/test_sf_express_settlement.py b/backend/tests/test_sf_express_settlement.py
new file mode 100644
index 00000000..8745f91d
--- /dev/null
+++ b/backend/tests/test_sf_express_settlement.py
@@ -0,0 +1,173 @@
+from datetime import datetime
+from decimal import Decimal
+
+import pytest
+
+from app.services.sf_express_settlement import (
+    assert_freight_mutable,
+    assert_settle_request,
+    freight_amount_label,
+    plan_fill_amount,
+    plan_settle,
+    resolve_header_for_save,
+)
+
+
+def test_new_sf_express_nulls_amount_and_requires_waybill():
+    plan = resolve_header_for_save(
+        transport_mode="sfkd",
+        existing_waybill=None,
+        existing_amount=None,
+        existing_settle_status=None,
+        waybill_supplied=True,
+        waybill_no="  SF001  ",
+        amount_supplied=True,
+        freight_amount=Decimal("88"),
+    )
+    assert plan.waybill_no == "SF001"
+    assert plan.freight_amount is None
+    assert plan.settle_status == "wjs"
+    assert plan.clear_settle_time is True
+    assert freight_amount_label("sfkd", plan.freight_amount) == "月结"
+
+
+def test_sf_express_rejects_blank_and_long_waybill():
+    with pytest.raises(ValueError, match="请填写顺丰单号"):
+        resolve_header_for_save(
+            transport_mode="sfkd",
+            existing_waybill=None,
+            existing_amount=None,
+            existing_settle_status=None,
+            waybill_supplied=True,
+            waybill_no="  ",
+            amount_supplied=True,
+            freight_amount=None,
+        )
+    with pytest.raises(ValueError, match="顺丰单号不能超过 64 个字符"):
+        resolve_header_for_save(
+            transport_mode="sfkd",
+            existing_waybill=None,
+            existing_amount=None,
+            existing_settle_status=None,
+            waybill_supplied=True,
+            waybill_no="S" * 65,
+            amount_supplied=False,
+            freight_amount=None,
+        )
+
+
+def test_non_sf_requires_amount_and_clears_waybill_and_settlement():
+    plan = resolve_header_for_save(
+        transport_mode="zxc",
+        existing_waybill="SF001",
+        existing_amount=None,
+        existing_settle_status="wjs",
+        waybill_supplied=False,
+        waybill_no=None,
+        amount_supplied=True,
+        freight_amount="12.3",
+    )
+    assert plan.waybill_no is None
+    assert plan.freight_amount == Decimal("12.30")
+    assert plan.settle_status is None
+    assert plan.clear_settle_time is True
+
+
+def test_other_mode_named_sf_still_requires_amount():
+    plan = resolve_header_for_save(
+        transport_mode="sf",
+        existing_waybill=None,
+        existing_amount=None,
+        existing_settle_status=None,
+        waybill_supplied=True,
+        waybill_no="",
+        amount_supplied=True,
+        freight_amount="12.30",
+    )
+    assert plan.freight_amount == Decimal("12.30")
+    assert plan.waybill_no is None
+    assert plan.settle_status is None
+
+
+def test_non_sf_rejects_missing_amount_and_negative_amount():
+    with pytest.raises(ValueError, match="请填写运费"):
+        resolve_header_for_save(
+            transport_mode="zxc",
+            existing_waybill=None,
+            existing_amount=None,
+            existing_settle_status=None,
+            waybill_supplied=True,
+            waybill_no="",
+            amount_supplied=True,
+            freight_amount=None,
+        )
+    with pytest.raises(ValueError, match="运费不能为负数"):
+        resolve_header_for_save(
+            transport_mode="zxc",
+            existing_waybill=None,
+            existing_amount=None,
+            existing_settle_status=None,
+            waybill_supplied=True,
+            waybill_no="",
+            amount_supplied=True,
+            freight_amount="-1",
+        )
+
+
+def test_settled_header_cannot_change():
+    with pytest.raises(ValueError, match="已结算的运费不能修改运输方式或顺丰单号"):
+        resolve_header_for_save(
+            transport_mode="sfkd",
+            existing_waybill="SF001",
+            existing_amount=None,
+            existing_settle_status="yjs",
+            waybill_supplied=True,
+            waybill_no="SF002",
+            amount_supplied=False,
+            freight_amount=None,
+        )
+
+
+def test_blank_settle_keeps_null_and_historical_amount():
+    now = datetime(2026, 10, 21, 12, 0, 0)
+    assert plan_settle(current_amount=None, submitted_amount="  ", now=now) == (
+        None,
+        "yjs",
+        now,
+    )
+    assert plan_settle(current_amount=Decimal("10.00"), submitted_amount=None, now=now) == (
+        Decimal("10.00"),
+        "yjs",
+        now,
+    )
+    amount, status, settle_time = plan_settle(
+        current_amount=Decimal("10.00"), submitted_amount="20", now=now
+    )
+    assert amount == Decimal("20.00")
+    assert status == "yjs"
+    assert settle_time == now
+
+
+def test_batch_settle_rejects_amount_and_fill_keeps_only_monthly_rows():
+    assert_settle_request(["a", "b"], None)
+    with pytest.raises(ValueError, match="批量结算不能填写运费"):
+        assert_settle_request(["a", "b"], "10")
+    with pytest.raises(ValueError, match="请选择运费批次"):
+        assert_settle_request([], None)
+    assert plan_fill_amount(
+        current_amount=None, settle_status="yjs", submitted_amount="6.5"
+    ) == Decimal("6.50")
+    with pytest.raises(ValueError, match="只有已结算且仍为月结的运费可以补填金额"):
+        plan_fill_amount(
+            current_amount=Decimal("1.00"), settle_status="yjs", submitted_amount="2"
+        )
+
+
+def test_settled_batch_cannot_delete_or_change_samples():
+    with pytest.raises(ValueError, match="已结算的运费不能删除"):
+        assert_freight_mutable("yjs", "delete")
+    with pytest.raises(ValueError, match="已结算的运费不能调整样本"):
+        assert_freight_mutable("yjs", "items")
+    assert_freight_mutable("wjs", "delete")
+    assert freight_amount_label("sfkd", Decimal("8")) == "8.00"
+    assert freight_amount_label("zxc", None) == ""