The Point Spread Function(PSF) of a pinhole imaging system for γ-Ray has been studied through Monte Carlo method.The PSFs under 0, 0.5, 1, 1.5 and 2 pixel bias five different conditions have been obtained and analyzed by fitting the simulating data with Gauss function, the corresponding Modulation Transfer Function(MTF) can be obtained, and the spatial resolutions under these five conditions are compared.As the results show, when the excursion is little, the PSF′ deviation obtained by Gauss function fitting will be less and can meet the requirement of accuracy, but when the excursion is larger, the fitting deviation will be more. In addition, a ideal baffle between pinhole and detector will reduce the error and improve the spatial resolution remarkably.