diff --git a/backend/app/services/oa_engine.py b/backend/app/services/oa_engine.py
index de0d2f25..030510c6 100644
--- a/backend/app/services/oa_engine.py
+++ b/backend/app/services/oa_engine.py
@@ -1449,16 +1449,29 @@ def start_instance(
     )
     db.add(inst)
 
-    for to_id in related_instance_ids or []:
-        to_id = (to_id or "").strip()
-        if not to_id:
-            continue
+    from app.services.oa_related_approval import (
+        assert_linkable_instance,
+        merge_relation_targets,
+    )
+
+    targets = merge_relation_targets(schema, data, related_instance_ids)
+    for field_key, to_id in targets:
+        target = (
+            db.query(OaInstance)
+            .filter(
+                OaInstance.id == to_id,
+                OaInstance.tenant_no == tenant_no,
+                OaInstance.del_flag == 0,
+            )
+            .first()
+        )
+        assert_linkable_instance(target, user_id=initiator_user_id, tenant_no=tenant_no)
         db.add(
             OaRelation(
                 id=generate_uuid(),
                 from_instance_id=inst.id,
                 to_instance_id=to_id,
-                field_key=None,
+                field_key=field_key,
                 tenant_no=tenant_no,
                 order_num=0,
                 remark=None,
diff --git a/backend/app/services/oa_related_approval.py b/backend/app/services/oa_related_approval.py
index fd69c10f..557cc5fb 100644
--- a/backend/app/services/oa_related_approval.py
+++ b/backend/app/services/oa_related_approval.py
@@ -117,6 +117,24 @@ def extract_related_approval_links(
     return links
 
 
+def merge_relation_targets(
+    schema: Any,
+    form_data: Optional[Dict[str, Any]],
+    legacy_ids: Optional[List[str]] = None,
+) -> List[Tuple[Optional[str], str]]:
+    """Returns (field_key|None, to_id) — form fields first, then legacy without key."""
+    links = extract_related_approval_links(schema, form_data)
+    result: List[Tuple[Optional[str], str]] = [(f, i) for f, i in links]
+    seen_ids = {i for _, i in links}
+    for x in legacy_ids or []:
+        tid = str(x or "").strip()
+        if not tid or tid in seen_ids:
+            continue
+        result.append((None, tid))
+        seen_ids.add(tid)
+    return result
+
+
 def assert_linkable_instance(inst: OaInstance, *, user_id: str, tenant_no: str) -> None:
     if not inst or inst.del_flag != 0 or inst.tenant_no != tenant_no:
         raise OaEngineError("关联的审批单不存在")
diff --git a/backend/tests/test_oa_related_approval.py b/backend/tests/test_oa_related_approval.py
index 75f946a7..74de55b6 100644
--- a/backend/tests/test_oa_related_approval.py
+++ b/backend/tests/test_oa_related_approval.py
@@ -3,6 +3,7 @@ import unittest
 from app.services.oa_related_approval import (
     extract_related_approval_links,
     flatten_form_fields_for_summary,
+    merge_relation_targets,
 )
 
 
@@ -57,5 +58,12 @@ class TestValidateLinkable(unittest.TestCase):
             assert_linkable_instance(inst, user_id="u1", tenant_no="t")
 
 
+class TestMergeRelationTargets(unittest.TestCase):
+    def test_merge_prefers_form_fields(self):
+        schema = {"rule": [{"type": "relatedApproval", "field": "r1"}]}
+        m = merge_relation_targets(schema, {"r1": ["a"]}, ["a", "b"])
+        self.assertEqual(m, [("r1", "a"), (None, "b")])
+
+
 if __name__ == "__main__":
     unittest.main()