when my QNX started, it swithced to one APP directly. How do I see the desktop?
You have not mentioned which version of QNX you are running. It also would be helpful to know if you are in text mode or running a GUI, presumably Photon. An application might be written to prevent you from getting out of the APP. Here is one thing to try.
CTRL-ALT [Big +]
or
CTRL-ALT. # where # is 1-9
In both text mode and Photon these can switch the console.
If none of these work, there are a number of alternative ways to getting into a system. If the network is setup, you may be able to invade a system from another QNX system. You also might be able to modify the startup script so as not to start the APP. Thare are various ways to go about this.