I want show my project main window in the middle of the screen . when i call "self.show()" then the window show in the middle of the screen .
First I would recommend against trying to force a window position on your users and let the system's window manager decide where it should go. If you really insist on positioning it yourself (perhaps you are programming for a kiosk), you can find some information here in a previous question on stackoverflow.
A slightly more elegant calculation for doing this is discussed here.
When doing this calculation, it is important that it is done at the correct time, after Qt has resized everything and just before it is shown on screen. One method that might help is to create a one-shot timer and do the screen positioning in the slot for the timer.