1. 修改数据库，在数据库中执行以下SQL代码（请把下面bbs_修改成你自己的表前缀后执行，如果是默认的是bbs_ 请直接复制）：

ALTER TABLE `bbs_post`
MODIFY COLUMN `message` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL AFTER `quotepid`,
MODIFY COLUMN `message_fmt` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL AFTER `message`;
ALTER TABLE `bbs_post_update_log`
MODIFY COLUMN `message` text CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL AFTER `reason`;
ALTER TABLE `bbs_thread`
MODIFY COLUMN `subject` char(128) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' AFTER `userip`;
ALTER TABLE `bbs_notice`
MODIFY COLUMN `message` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL AFTER `type`;
ALTER TABLE `bbs_modlog`
MODIFY COLUMN `subject` char(32) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' AFTER `pid`,
MODIFY COLUMN `comment` char(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' AFTER `subject`;

2. 修改配置文件bbs/conf/conf.php

   把15行和32行的 'charset' => 'utf8', 修改为 'charset' => 'utf8mb4',

3. OK，去试试吧

补充：utf8mb4_unicode_ci 也可以使用 utf8mb4_general_ci