It looks that you have to change system locale LC_CTYPE to zh_CN.UFT-8 to enable input Chinese characters in Emacs. See 英文ubuntu系统中emacs输入中文. But this will cause the GUI to Chinese, which make search difficult.
So when you want to write Chinese characters in Org mode, use vi to open the org file, edit it. Both Emacs and vi use ":e" to reload file (if emacs have no Evil, use C-x C-v). That is to say, after add some Chinese characters in vi, save file, switch to Emacs, use :e to reload file.
Note: Use :diffthis to compare 2 buffer in vi, :diffoff to quit comparison mode. Google "vim reload file" for details.