diff --git a/lib/api_material.js b/lib/api_material.js
index 53c2a21..726b499 100644
--- a/lib/api_material.js
+++ b/lib/api_material.js
@@ -7,9 +7,6 @@ const formstream = require('formstream');
 
 const { postJSON } = require('./util');
 
-//永久上传素材的url的前缀
-const prefixUrl = 'https://api.weixin.qq.com/cgi-bin/';
-
 /**
  * 上传永久素材，分别有图片（image）、语音（voice）、和缩略图（thumb）
  * 详情请见：<http://mp.weixin.qq.com/wiki/14/7e6c03263063f4813141c3e17dd4350a.html>
@@ -32,7 +29,7 @@ exports.uploadMaterial = async function (filepath, type) {
   var stat = await fs.stat(filepath);
   var form = formstream();
   form.file('media', filepath, path.basename(filepath), stat.size);
-  var url = prefixUrl + 'material/add_material?access_token=' + accessToken + '&type=' + type;
+  var url = this.prefix + 'material/add_material?access_token=' + accessToken + '&type=' + type;
   var opts = {
     dataType: 'json',
     method: 'POST',
@@ -75,7 +72,7 @@ exports.uploadVideoMaterial = async function (filepath, description) {
   var form = formstream();
   form.file('media', filepath, path.basename(filepath), stat.size);
   form.field('description', JSON.stringify(description));
-  var url = prefixUrl + 'material/add_material?access_token=' + accessToken + '&type=video';
+  var url = this.prefix + 'material/add_material?access_token=' + accessToken + '&type=video';
   var opts = {
     dataType: 'json',
     method: 'POST',
@@ -194,10 +191,10 @@ exports.getMaterial = async function (mediaId) {
  */
 exports.removeMaterial = async function (mediaId) {
   const { accessToken } = await this.ensureAccessToken();
-  var prefix = 'https://api.weixin.qq.com/cgi-bin/';
-  var url = prefix + 'material/del_material?access_token=' + accessToken;
+  var url = this.prefix + 'material/del_material?access_token=' + accessToken;
   return this.request(url, postJSON({'media_id': mediaId}));
 };
+
 /**
  * 获取素材总数
  * 详情请见：<http://mp.weixin.qq.com/wiki/16/8cc64f8c189674b421bee3ed403993b8.html>
@@ -219,8 +216,7 @@ exports.removeMaterial = async function (mediaId) {
  */
 exports.getMaterialCount = async function () {
   const { accessToken } = await this.ensureAccessToken();
-  var prefix = 'https://api.weixin.qq.com/cgi-bin/';
-  var url = prefix + 'material/get_materialcount?access_token=' + accessToken;
+  var url = this.prefix + 'material/get_materialcount?access_token=' + accessToken;
   return this.request(url, {dataType: 'json'});
 };
 
@@ -253,8 +249,7 @@ exports.getMaterialCount = async function () {
  */
 exports.getMaterials = async function (type, offset, count) {
   const { accessToken } = await this.ensureAccessToken();
-  var prefix = 'https://api.weixin.qq.com/cgi-bin/';
-  var url = prefix + 'material/batchget_material?access_token=' + accessToken;
+  var url = this.prefix + 'material/batchget_material?access_token=' + accessToken;
   var data = {
     type: type,
     offset: offset,