diff --git a/apps/backend/src/routes/admin/worker-platform/orders.ts b/apps/backend/src/routes/admin/worker-platform/orders.ts
index 1fc67840..81da73c4 100644
--- a/apps/backend/src/routes/admin/worker-platform/orders.ts
+++ b/apps/backend/src/routes/admin/worker-platform/orders.ts
@@ -31,6 +31,7 @@ import {
   updateAdminWorkOrderPaymentAmount,
   updateAdminWorkOrderSharing,
 } from '../../../services/worker-platform/index.js'
+import { buildWorkOrderPaymentAmountAuditData } from '../../../services/worker-platform/work-order-payment-amount.js'
 import {
   createJsonHandler,
   requireAdminPermission,
@@ -251,20 +252,14 @@ router.put(
       errorMessage: '修改用户付款金额失败',
       scope: '[admin/worker-platform/orders/:workOrderId/payment-amount]',
       audit: (req, data) => {
-        const result = (data || {}) as {
-          previousAmountFen?: number | null
-          amountFen?: number
-          reason?: string
-        }
+        // 金额未变化时服务层不会写工单事件，这里也不留下一条“已更新”的审计记录。
+        const auditData = buildWorkOrderPaymentAmountAuditData(data)
+        if (!auditData) return null
         return {
           action: 'work_order_payment_amount_updated',
           targetType: 'work_order',
           targetId: String(req.params.workOrderId || ''),
-          data: {
-            previousAmountFen: result.previousAmountFen ?? null,
-            amountFen: result.amountFen ?? null,
-            reason: result.reason || '',
-          },
+          data: auditData,
         }
       },
     },
diff --git a/apps/backend/src/services/worker-platform/work-order-payment-amount.test.ts b/apps/backend/src/services/worker-platform/work-order-payment-amount.test.ts
index 00495eea..db5e5c4d 100644
--- a/apps/backend/src/services/worker-platform/work-order-payment-amount.test.ts
+++ b/apps/backend/src/services/worker-platform/work-order-payment-amount.test.ts
@@ -2,6 +2,7 @@ import assert from 'node:assert/strict'
 import test from 'node:test'
 
 import {
+  buildWorkOrderPaymentAmountAuditData,
   mergeUserPaymentAmountIntoMaterial,
   normalizeUserPaymentAmountFen,
   resolveDisplayedUserPaymentAmountFen,
@@ -87,3 +88,38 @@ test('mergeUserPaymentAmountIntoMaterial：兼容 JSON 字符串与异常结构'
     source: { orderAmountFen: 1234 },
   })
 })
+
+test('buildWorkOrderPaymentAmountAuditData：未变化不写审计，变化时保留原值', () => {
+  assert.equal(
+    buildWorkOrderPaymentAmountAuditData({
+      unchanged: true,
+      previousAmountFen: 1000,
+      amountFen: 1000,
+      reason: '',
+    }),
+    null,
+    '未变化提交不能留下「已更新」审计',
+  )
+
+  assert.deepEqual(
+    buildWorkOrderPaymentAmountAuditData({
+      unchanged: false,
+      previousAmountFen: 1000,
+      amountFen: 1234,
+      reason: ' 修正 ',
+    }),
+    { previousAmountFen: 1000, amountFen: 1234, reason: '修正' },
+  )
+
+  assert.deepEqual(
+    buildWorkOrderPaymentAmountAuditData({
+      unchanged: false,
+      previousAmountFen: null,
+      amountFen: 1234,
+    }),
+    { previousAmountFen: null, amountFen: 1234, reason: '' },
+    '原标题金额缺失时保留 null，不能误写成 0',
+  )
+
+  assert.equal(buildWorkOrderPaymentAmountAuditData(null), null)
+})
diff --git a/apps/backend/src/services/worker-platform/work-order-payment-amount.ts b/apps/backend/src/services/worker-platform/work-order-payment-amount.ts
index f8d7dbc6..2be94b20 100644
--- a/apps/backend/src/services/worker-platform/work-order-payment-amount.ts
+++ b/apps/backend/src/services/worker-platform/work-order-payment-amount.ts
@@ -78,3 +78,39 @@ export function mergeUserPaymentAmountIntoMaterial(
   }
   return { ...material, source: nextSource }
 }
+
+export type WorkOrderPaymentAmountAuditData = {
+  previousAmountFen: number | null
+  amountFen: number | null
+  reason: string
+}
+
+function toAmountFenOrNull(value: unknown): number | null {
+  if (value === null || value === undefined || value === '') return null
+  const parsed = Number(value)
+  return Number.isFinite(parsed) ? parsed : null
+}
+
+/**
+ * 后台审计载荷：金额未变化时不写「已更新」审计。
+ *
+ * 前端已拦截未变化提交，但接口仍可能被直接调用或重放；此时服务层不会写
+ * payment_amount_updated 工单事件，审计也不能留下一条原金额等于新金额的修改记录。
+ */
+export function buildWorkOrderPaymentAmountAuditData(
+  data: unknown,
+): WorkOrderPaymentAmountAuditData | null {
+  if (!data || typeof data !== 'object' || Array.isArray(data)) return null
+  const result = data as {
+    previousAmountFen?: unknown
+    amountFen?: unknown
+    reason?: unknown
+    unchanged?: unknown
+  }
+  if (result.unchanged === true) return null
+  return {
+    previousAmountFen: toAmountFenOrNull(result.previousAmountFen),
+    amountFen: toAmountFenOrNull(result.amountFen),
+    reason: String(result.reason || '').trim(),
+  }
+}