The FPN is an important part in CCD noise. The positions of the FPN in CCD are fixed, and the magnitude follows some rules. Through measuring the positions and magnitude of the FPN beforehand, we could eliminate the FPN in the image processing afterward. The causes and forms of the FPN are analyzed. The FPN of the science grad CCD Alta U9000 is introduced. The relationship between the FPN of U9000 and the exposure time is determined with the shutter closed. And the FPN of images taken in another experiment is eliminated. The experiment results well.