While screen is rendering (in render func) ,then need to change to another big scene.It need to preparing resource (such as load X file) to render new scene. How to ??? I trying to go to sub function to load resource when global variable indicate it is going to new scenem,but it not work. It work like rendering + loading at the same time.(cause slow mouse pointing)

You can do this by making use of multi-threading (i.e. have a separate resource loading/management thread) that executes ''alongside'' your main rendering code. Of course this presents further complications along the lines of resource sharing, mutual exclusivity, object sharing, etc., etc.

I think the best approach is to simply pre-load as much as possible so that you have the majority (if not all) of the next scene in memory and ready for display.