Abstract: Numerical simulation method could be used to investigate the possibility and effectiveness of phase contrast imaging for certain testing objects, moreover, to optimize imaging parameters. For X-ray in-line phase-contrast imaging system, principle of numerical simulation and the implementation method were analyzed first, and simulation for testing objects with round shape and strip shape were carried out afterward. After comparison of the simulation results, we know that the discretization size of testing object affects, to a large extent, the accuracy of numerical simulation results. The reason is that the accuracy of optical transform function (OTF) in the phase contrast imaging is determined by the discretization size of testing object. To guarantee the accuracy of simulation results, we should ensure no evident distortions exist in discretization of OTF and the discretization points are continuous. For an accurate simulation, the discrete size of OTF is not fixed, which should match the size of the testing object.