diff --git a/web-admin/src/utils/protocol-sample-payment.test.ts b/web-admin/src/utils/protocol-sample-payment.test.ts
index ddae9126..66065e0b 100644
--- a/web-admin/src/utils/protocol-sample-payment.test.ts
+++ b/web-admin/src/utils/protocol-sample-payment.test.ts
@@ -15,7 +15,9 @@ import {
   applyConsolidateRemainingToPayment,
   applyExpandRemainingToInstallment,
   availableSamplePayModes,
-  samplePaymentHasConfirmedTransfer
+  samplePaymentHasConfirmedTransfer,
+  calcLastPeriodDueAmount,
+  shouldAutoFillLastPeriodDue
 } from './protocol-sample-payment'
 
 describe('parsePreferentialPrice', () => {
@@ -331,3 +333,42 @@ describe('samplePaymentHasConfirmedTransfer', () => {
     )
   })
 })
+
+describe('calcLastPeriodDueAmount', () => {
+  it('last = contract − sum(priors), cents remainder on last', () => {
+    assert.equal(calcLastPeriodDueAmount(100, [33.33, 33.33]), 33.34)
+    assert.equal(calcLastPeriodDueAmount(20000, [10000, 5000]), 5000)
+  })
+  it('returns null when contract invalid', () => {
+    assert.equal(calcLastPeriodDueAmount(null, [1000]), null)
+    assert.equal(calcLastPeriodDueAmount(NaN, [1000]), null)
+  })
+  it('returns null when all priors empty', () => {
+    assert.equal(calcLastPeriodDueAmount(20000, [null, '', undefined]), null)
+  })
+  it('treats empty slot as 0 when at least one prior filled', () => {
+    assert.equal(calcLastPeriodDueAmount(20000, [10000, null, '']), 10000)
+  })
+  it('clamps negative result to 0', () => {
+    assert.equal(calcLastPeriodDueAmount(1000, [800, 800]), 0)
+  })
+})
+
+describe('shouldAutoFillLastPeriodDue', () => {
+  it('blocks when override or locked', () => {
+    assert.equal(
+      shouldAutoFillLastPeriodDue({ lastManualOverride: true, lastLocked: false }),
+      false
+    )
+    assert.equal(
+      shouldAutoFillLastPeriodDue({ lastManualOverride: false, lastLocked: true }),
+      false
+    )
+  })
+  it('allows when neither override nor locked', () => {
+    assert.equal(
+      shouldAutoFillLastPeriodDue({ lastManualOverride: false, lastLocked: false }),
+      true
+    )
+  })
+})
diff --git a/web-admin/src/utils/protocol-sample-payment.ts b/web-admin/src/utils/protocol-sample-payment.ts
index 795abcdb..d715963c 100644
--- a/web-admin/src/utils/protocol-sample-payment.ts
+++ b/web-admin/src/utils/protocol-sample-payment.ts
@@ -409,3 +409,34 @@ export function applyExpandRemainingToInstallment<T extends Record<string, any>>
     })
   }
 }
+
+/** 分期：最后一期应缴 = 合同 − 前 N−1 期；无法计算返回 null；负值钳为 0 */
+export function calcLastPeriodDueAmount(
+  contractAmount: number | null | undefined,
+  priorAmounts: Array<number | string | null | undefined>
+): number | null {
+  // Number(null) === 0；须先排除空值，否则会误算尾期
+  if (contractAmount == null) return null
+  const contract = Number(contractAmount)
+  if (!Number.isFinite(contract)) return null
+  if (!priorAmounts.length) return null
+  let anyFilled = false
+  let sumCents = 0
+  for (const raw of priorAmounts) {
+    if (raw === null || raw === undefined || raw === '') continue
+    const n = Number(raw)
+    if (!Number.isFinite(n)) continue
+    anyFilled = true
+    sumCents += Math.round(n * 100)
+  }
+  if (!anyFilled) return null
+  const last = Math.round(contract * 100 - sumCents) / 100
+  return last < 0 ? 0 : last
+}
+
+export function shouldAutoFillLastPeriodDue(opts: {
+  lastManualOverride: boolean
+  lastLocked: boolean
+}): boolean {
+  return !opts.lastManualOverride && !opts.lastLocked
+}