diff --git a/lib/payment.js b/lib/payment.js
index 5a7a0cc..70c5e1c 100644
--- a/lib/payment.js
+++ b/lib/payment.js
@@ -251,7 +251,7 @@ Payment.prototype._signedQuery = function(url, params, options, callback) {
   }
 
   var request = (options.https ? this._httpsRequest : this._httpRequest).bind(this);
-  request(url, this._buildXml(params), function(err, body) {
+  request(url, this.buildXml(params), function(err, body) {
     if (err) {
       return callback(err);
     }
@@ -347,7 +347,7 @@ Payment.prototype._parseCsv = function(text) {
   };
 };
 
-Payment.prototype._buildXml = function(obj) {
+Payment.prototype.buildXml = function(obj) {
   var builder = new xml2js.Builder({
     allowSurrogateChars: true
   });
@@ -469,7 +469,8 @@ Payment.prototype._generateNonceStr = function(length) {
 if (global.Promise) {
   for (let key in Payment.prototype) {
     let func = Payment.prototype[key]
-    if (typeof func == 'function' && key.indexOf('_') !== 0) {
+    let syncFuncs = ['buildXml']
+    if (typeof func == 'function' && key.indexOf('_') !== 0 && syncFuncs.indexOf(key) === -1) {
       Payment.prototype[key] = function () {
         let args = Array.prototype.slice.call(arguments)
         let originCallback = args[args.length - 1]
diff --git a/package.json b/package.json
index 4bfb838..814d47b 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "wechat-pay",
-  "version": "0.3.1",
+  "version": "0.3.2",
   "description": "wechat payment api for document v3.3.5",
   "main": "index.js",
   "scripts":
@@ -31,4 +31,4 @@
     "underscore": "^1.7.0",
     "xml2js": "^0.4.4"
   }
-}
\ No newline at end of file
+}