diff --git a/documentation20/cn/12.taos-sql/docs.md b/documentation20/cn/12.taos-sql/docs.md
index 31b639be91d08e78a4927ef4bf244367358c828e..f270f397354f4658fe8a65398872eebaebc7512b 100755
--- a/documentation20/cn/12.taos-sql/docs.md
+++ b/documentation20/cn/12.taos-sql/docs.md
@@ -2150,7 +2150,7 @@ TDengine支持针对数据的聚合查询。提供支持的聚合和选择函数
     说明：
     
       如果输入值为NULL，输出值为NULL。
-      输入参数pos可以为正数，也可以为负数。如果pos是正数，表示从开始往后抽取子串。如果pos为负数，表示从结尾往前抽取字符串。如果输入参数len被忽略，返回的子串包含从pos开始的整个字串。
+      输入参数pos可以为正数，也可以为负数。如果pos是正数，表示开始位置从字符串开头正数计算。如果pos为负数，表示开始位置从字符串结尾倒数计算。如果输入参数len被忽略，返回的子串包含从pos开始的整个字串。
       该函数可以应用在普通表和超级表上。
       该函数适用于内层查询和外层查询。
       版本2.6.0.x后支持
diff --git a/documentation20/en/12.taos-sql/docs.md b/documentation20/en/12.taos-sql/docs.md
index 8b40da8c7609290224c40d383848c70295f19274..b6ed11c7cf401b8a234f7f1d06a90ed61dc8e9b9 100755
--- a/documentation20/en/12.taos-sql/docs.md
+++ b/documentation20/en/12.taos-sql/docs.md
@@ -1703,7 +1703,7 @@ TDengine supports aggregations over data, they are listed below:
     Notes:
     
       Returns NULL when input is NULL. 
-      Input pos can be negative or positive. If it is a positive number, this function extracts from the beginning of the string. If it is a negative number, this function extracts from the end of the string
+      Input pos can be negative or positive. If it is a positive number, the beginning of the substring is pos characters from the beginning of the string. If it is a negative number, the beginning of the substring is pos characters from the end of the string
       If input len is omitted, the output is whole substring starting from pos.
       This function applies to normal table, child table and super table
       This function applies to bother out query and inner query