Abstract: In the study of X ray coded aperture imaging system, it's known that the point spread function(PSF) decides the performance of a imaging system. MTF can be deduced by PSF and the filter used in image reconstruction can be made thereby, so the accuracy of PSF directly affects the performance of reconstructed image. In this paper, based on the scalar theory， PSF is obtained theoretically under the condition of considering diffraction effects. The filter of the optical system is fabricated based on PSF. The filtering processing for the frequency distribution of coded overlapping image was done with Wiener filtering function and then the reconstructed image was obtained through the inverse Fourier transformation.