I installed a Chinese font simsun.ttc, then all the pure English fonts seem attached to simsun.
For example, I am using Dejavu sans mono in Konsole. Before I install simsun, Dejavu sans mono is attached to wqy-zenhei.ttc.
So, in this case, English is shown in Dejavu sans mono and Chinese is shown in wqy-zenhei.
But now English is shown in Dejavu sans mono and Chinese is shown in simsun.
How to detach English fonts from simsun without uninstall it?

I believe that's a FreeType setting and has nothing to do with any KDE component.
If I recall correctly, you have to find the font substitution rules (a config file somewhere in /etc or so) and modify them by hand.