方法一：
1.关闭mysql
# service mysqld stop
2.屏蔽权限
# mysqld_safe --skip-grant-table
屏幕出现： Starting demo from .....
3.新开起一个终端输入
# mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD(‘newpassword‘) where USER=‘root‘;
mysql> FLUSH PRIVILEGES;//记得要这句话，否则如果关闭先前的终端，又会出现原来的错误
mysql> \q
方法二：
1.关闭mysql
# service mysqld stop
2.屏蔽权限
# mysqld_safe --skip-grant-table
屏幕出现： Starting demo from .....
3.新开起一个终端输入
# mysql -u root mysql
mysql> delete from user where USER=‘‘;
mysql> FLUSH PRIVILEGES;//记得要这句话，否则如果关闭先前的终端，又会出现原来的错误
mysql> \q
Mysql-ERROR 1044 (42000): Access denied for user ''@'localhost' to database 'mysql'
原文：http://www.cnblogs.com/hwaggLee/p/4592009.html