问题前提：mysql版本>=4.1

1、修改mysql目录下的my.ini
#default-character-set=latin1
default-character-set=utf8
2、修改wp-includes/wp-db.php
在对象wpdb中添加函数set_charset($charset)：
class wpdb{

 function set_charset($charset)
 {
  // check mysql version first.
  $serverVersion = mysql_get_server_info($this->dbh);
  $version = explode('.', $serverVersion);
  if ($version[0] < 4) return;
  // check if utf8 support was compiled in
  $result = mysql_query("SHOW CHARACTER SET like 'utf8'", $this->dbh);
  if (mysql_num_rows($result) <= 0)
   return;
  if ($charset == 'utf-8' || $charset == 'UTF-8')
   $charset = 'utf8';
  @mysql_query("SET NAMES '$charset'", $this->dbh);
 }
}

3、并修改wordpressmu根目录下的wp-settings.php:
require (ABSPATH . WPINC . '/vars.php');

$wpdb->set_charset(get_bloginfo('charset')); // add at this

4、重新创建wordpressmu的数据库（重新安装wordpressmu）。