diff --git a/README.md b/README.md
index 013cc3a..da16e9e 100644
--- a/README.md
+++ b/README.md
@@ -55,12 +55,13 @@ WxPay.extra_rest_client_options = {timeout: 2, open_timeout: 3}
 If you need to use sandbox mode.
 
 ```ruby
-result = WxPay::Service.get_sandbox_signkey('YOUR_MCH_ID')
-WxPay.key = result['sandbox_signkey']
 WxPay.appid = 'YOUR_APPID'
 WxPay.mch_id = 'YOUR_MCH_ID'
 WxPay.debug_mode = true # default is `true`
 WxPay.sandbox_mode = true # default is `false`
+result = WxPay::Service.get_sandbox_signkey
+WxPay.key = result['sandbox_signkey']
+
 ```
 
 Note: You should create your APIKEY (Link to [微信商户平台](https://pay.weixin.qq.com/index.php/home/login)) first if you haven't, and pay attention that **the length of the APIKEY should be 32**.
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