diff --git a/frontend/src/views/admin/SettingsView.vue b/frontend/src/views/admin/SettingsView.vue
index 83f166b8f2..271ef3191f 100644
--- a/frontend/src/views/admin/SettingsView.vue
+++ b/frontend/src/views/admin/SettingsView.vue
@@ -9559,7 +9559,11 @@ async function saveSettings() {
       password_reset_enabled: form.password_reset_enabled,
       totp_enabled: form.totp_enabled,
       session_binding_enabled: form.session_binding_enabled,
-      audit_log_retention_days: form.audit_log_retention_days,
+      // 清空数字框时 v-model.number 会得到空串，后端 int 字段解析空串会 400 拒绝整次保存；
+      // 空/非法值回退默认 180（与后端 parseAuditLogRetentionDays("") 语义一致，0 仍表示永久保留）。
+      audit_log_retention_days: Number.isFinite(form.audit_log_retention_days)
+        ? form.audit_log_retention_days
+        : 180,
       login_agreement_enabled: form.login_agreement_enabled,
       login_agreement_mode: form.login_agreement_mode,
       login_agreement_updated_at: form.login_agreement_updated_at,