【原因】

原來MySQL5.6.6版本之後增加了密碼強度驗證外掛validate_password，相關引數設定的較為嚴格。
使用了該外掛會檢查設定的密碼是否符合當前設定的強度規則，若不滿足則拒絕設定。影響的語句和函式有：create user,grant,set password,password(),old password。

【解決】

1） 檢視mysql全域性引數配置

該問題其實與mysql的validate_password_policy的值有關。
檢視一下msyql密碼相關的幾個全域性引數：

mysql> select @@validate_password_policy;
+----------------------------+
| @@validate_password_policy |
+----------------------------+
| MEDIUM                     |
+----------------------------+
1 row in set (0.00 sec)
 
 
mysql> SHOW VARIABLES LIKE 'validate_password%';
+--------------------------------------+--------+
| Variable_name                        | Value  |
+--------------------------------------+--------+
| validate_password.dictionary_file    |        |
| validate_password.length             | 8      |
| validate_password.mixed_case_count   | 1      |
| validate_password.number_count       | 1      |
| validate_password.policy             | MEDIUM |
| validate_password.special_char_count | 1      |
+--------------------------------------+--------+
6 rows in set (0.08 sec)
 

2）引數解釋

validate_password.dictionary_file
外掛用於驗證密碼強度的字典檔案路徑。

validate_password.length
密碼最小長度，引數預設為8，它有最小值的限制，最小值為：validate_password_number_count + validate_password_special_char_count + (2 * validate_password_mixed_case_count)

validate_password.mixed_case_count
密碼至少要包含的小寫字母個數和大寫字母個數。

validate_password.number_count

3）修改mysql引數配置

    mysql> set global validate_password.policy=0;
    Query OK, 0 rows affected (0.05 sec)
     
    mysql>
    mysql>
    mysql> set global validate_password.mixed_case_count=0;
    Query OK, 0 rows affected (0.00 sec)
     
    mysql> set global validate_password.number_count=3;
    Query OK, 0 rows affected (0.00 sec)
     
    mysql> set global validate_password.special_char_count=0;
    Query OK, 0 rows affected (0.00 sec)
     
    mysql> set global validate_password.length=3;
    Query OK, 0 rows affected (0.00 sec)
     
    mysql> SHOW VARIABLES LIKE 'validate_password%';
    +--------------------------------------+-------+
    | Variable_name                        | Value |
    +--------------------------------------+-------+
    | validate_password.dictionary_file    |       |
    | validate_password.length             | 3     |
    | validate_password.mixed_case_count   | 0     |
    | validate_password.number_count       | 3     |
    | validate_password.policy             | LOW   |
    | validate_password.special_char_count | 0     |
    +--------------------------------------+-------+
    6 rows in set (0.00 sec)

4）修改密碼：

    ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'new-password';

    Query OK, 0 rows affected, 1 warning (0.00 sec)

OK了。