diff --git a/sql-statements/sql-statement-create-index.md b/sql-statements/sql-statement-create-index.md
index 19d4efecc512c..b54b628cde16a 100644
--- a/sql-statements/sql-statement-create-index.md
+++ b/sql-statements/sql-statement-create-index.md
@@ -111,6 +111,18 @@ Query OK, 0 rows affected (0.31 sec)
 
 ## Expression index
 
+> **Note:**
+>
+> Expression index is still an experimental feature. It is **NOT** recommended that you use it in the production environment.
+
+To use this feature, make the following setting in [TiDB Configuration File](/tidb-configuration-file.md#allow-expression-index-new-in-v400):
+
+{{< copyable "sql" >}}
+
+```sql
+allow-expression-index = true
+```
+
 TiDB can build indexes not only on one or more columns in a table, but also on an expression. When queries involve expressions, expression indexes can speed up those queries.
 
 Take the following query as an example:
diff --git a/tidb-configuration-file.md b/tidb-configuration-file.md
index 053f12ebfbcfc..291f3a6812c9a 100644
--- a/tidb-configuration-file.md
+++ b/tidb-configuration-file.md
@@ -612,4 +612,9 @@ For pessimistic transaction usage, refer to [TiDB Pessimistic Transaction Mode](
 
 ## experimental
 
-The `experimental` section, introduced in v3.1.0, describes configurations related to the experimental features of TiDB. Currently, this section has no configuration item.
+The `experimental` section, introduced in v3.1.0, describes configurations related to the experimental features of TiDB.
+
+### `allow-expression-index` <span class="version-mark">New in v4.0.0</span>
+
+- Determines whether to create the expression index.
+- Default value: false
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