Abstract: Tradition systems of beam drift measurement always need measuring at near field and far field respectively, so they are complex and non-real time. A new theory of drift measurement based on the location change of diffraction spot was proposed, which was not applied. For verifying the theory and building a predigested measurement system, the design of the key elements needed to be simulated. Based on the imaging theory of defocus grating, the model of beam transmission was built by Matlab. The simulation result was consistent with the theory. Finally, the relation between the key parameters of the grating and the maximum drift value was simulated and analyzed. The Results shows that the best design parameters can be obtained while the off-axis value of defocus grating is 15 mm and the focus length ratio of defocus grating to short focus lens is 6 under the condition of the imaging unit 1 cm2 and the focus of short lens 12 cm.