Charge coupled device (CCD) is a pivotal module of imaging system. The point spread function (PSF) of CCD is an important part of the PSF of imaging system. A backilluminated , partially depleted CCD is studied. The movements of the carriers in the CCD are analysed. The PSF are simulated based on MonteCarlo method. The simulative PSF can well match analytical PSF which are also computed. The main characteristics of CCD are analyzed，such as responsibility,linearity and modulation transfer function. The influences to PSF caused by the change of wavelength and fieldfree region are simulated, which show that better PSF can be attained by smaller fieldfree region or longer wavelength.