diff --git a/dbms/src/Functions/FunctionsTiDBConversion.h b/dbms/src/Functions/FunctionsTiDBConversion.h
index a1a32c9ac54..954cc2e8d49 100644
--- a/dbms/src/Functions/FunctionsTiDBConversion.h
+++ b/dbms/src/Functions/FunctionsTiDBConversion.h
@@ -1011,9 +1011,9 @@ struct TiDBConvertToDecimal
         size_t pos = 0;
         if (decimal_parts.int_part.data[pos] == '+' || decimal_parts.int_part.data[pos] == '-')
         {
-            pos++;
             if (decimal_parts.int_part.data[pos] == '-')
                 is_negative = true;
+            pos++;
         }
         Int256 max_value = DecimalMaxValue::Get(prec);
 
diff --git a/tests/fullstack-test/expr/cast_string_as_decimal.test b/tests/fullstack-test/expr/cast_string_as_decimal.test
new file mode 100644
index 00000000000..13d908ee6a5
--- /dev/null
+++ b/tests/fullstack-test/expr/cast_string_as_decimal.test
@@ -0,0 +1,22 @@
+mysql> drop table if exists test.t
+mysql> create table test.t(a char(10))
+mysql> alter table test.t set tiflash replica 1
+mysql> insert into test.t values('-123')
+mysql> insert into test.t values('2006-01')
+
+func> wait_table test t
+
+mysql> set tidb_enforce_mpp=1; select cast(a as decimal) from test.t;
++--------------------+
+| cast(a as decimal) |
++--------------------+
+|               -123 |
+|               2006 |
++--------------------+
+mysql> set @@tidb_isolation_read_engines='tikv'; select cast(a as decimal) from test.t;
++--------------------+
+| cast(a as decimal) |
++--------------------+
+|               -123 |
+|               2006 |
++--------------------+
\ No newline at end of file