Based on the principle of SLM beam shaping, the structural characteristics of three type grating: binary grating, blazed grating and sinusoidal grating are analyzed. Phase masks with different periods of the three type gratings are designed according to the performance that affect the quality of shaping beam, taking square form as an example. Imaging and machined experiments show that all the phase mask formed by loading three type gratings on SLM can shape the gaussian circular beam into a flat-topped square, but the shaping effect of blazed gratings is obviously better than that of the other two gratings. When the grating period was 120 μm, the local energy utilization rate was 91.86%, and the RMSE was 0.039. Using blazed grating, the shaped beam has high energy efficiency and good uniformity.