1、安装Fcitx,打开shell,输入命令
sudo apt-get install fcitx

我之前就已经安装好了
2、进入System Setting(系统设置)

3、进入Region & Language —>Manage Installed Languages。如果出现The language support is not installed completely,点击Remind Me Later

4、点击Install/Remove Languages…
5、找到Chinese(simplified)并勾选,Apply后输入密码,稍等一会