The gtk2 doesn't support 'Over The Spot' input style, so I can not use xcin with it....

xcin can work with gtk2 in root style (but not over the spot)
If you want to use xcin in gtk2, you should
export GTK_IM_MODULE=xim
then you can use xcin in gtk2

I know that, but it kinda stupid to use gtk2 with xcin right now, I think we should wait til xcin support the "on the spot" input style.