This work is written in English, because theree are no input tools on my raspberry pi devices. 
So i install it by typing the following commands:

sudo apt-get install scim-pinpin

But it doesn't work. Maybe it needs updating software source.

sudo apt-get update

Finally, it works.
然后按ctrl + 空格，就可以了。
我的树莓派里安装的 debian 系列的系统，默认截图工具是scropt 截图只能用命令行，有点不方便。