diff --git a/lib/wx_pay/service.rb b/lib/wx_pay/service.rb
index 69e491c..cb1179b 100644
--- a/lib/wx_pay/service.rb
+++ b/lib/wx_pay/service.rb
@@ -454,7 +454,7 @@ module WxPay
 
       def xmlify_payload(params, sign_type = WxPay::Sign::SIGN_TYPE_MD5)
         sign = WxPay::Sign.generate(params, sign_type)
-        "<xml>#{params.except(:key).map { |k, v| "<#{k}>#{v}</#{k}>" }.join}<sign>#{sign}</sign></xml>"
+        "<xml>#{params.except(:key).sort.map { |k, v| "<#{k}>#{v}</#{k}>" }.join}<sign>#{sign}</sign></xml>"
       end
 
       def make_payload(params, sign_type = WxPay::Sign::SIGN_TYPE_MD5)
--
libgit2 0.21.0