WeChat Pay unified issued an order with an error in the signature of the interface.

Wechat"s unified API for issuing orders always reports signature errors!
I use Wechat"s official signature to verify that the signature result is the same. Is there any good way to verify what caused the signature error?

here is the xml: passed by my calling API

<xml>
    <mch_id>1423xxxxxxx7402</mch_id>
    <nonce_str><![CDATA[5b6fc9879716a]]></nonce_str>
    <body><![CDATA[deal]]></body>
    <out_trade_no><![CDATA[SN2018081213435352966711]]></out_trade_no>
    <total_fee>20000</total_fee>
    <notify_url><![CDATA[http://xxx.com/api/payment/notify]]></notify_url>
    <trade_type><![CDATA[JSAPI]]></trade_type>
    <openid><![CDATA[oV8-KxxxxxxYuARnthpFiQec]]></openid>
    <spbill_create_ip><![CDATA[36.xx.xxx.252]]></spbill_create_ip>
    <appid><![CDATA[wxxxxxxxe73053]]></appid>
    <sign><![CDATA[0400701EF7F990D9652BBxxE5E8D8ACB]]></sign>
</xml>

{
    "mch_id": 1423xxxxxxx7402,
    "nonce_str": "5b6fc9879716a",
    "body": "deal",
    "out_trade_no": "SN2018081213435352966711",
    "total_fee": 20000,
    "notify_url": "http://xxx.com/api/payment/notify",
    "trade_type": "JSAPI",
    "openid": "oV8-KxxxxxxYuARnthpFiQec",
    "spbill_create_ip": "36.xx.xxx.252",
    "appid": "wxxxxxxxe73053",
    "sign": "0400701EF7F990D9652BBxxE5E8D8ACB"
}