The effect the diffractive optical characteristic by the surface bending of two-dimensional grating light modulator (GLM) is disaused. Based on the measured surface structure data, an array model was built, and the effect on diffractive light distribution in spectrum plane with different bendings was calculated by Matlab software. The relation between bending and contrast was deduced. The result indicates that the surface bending can affect the image contrast greatly in condition of darkness. If the bending in center is reduced less than 0.1λ, the contrast will be larger than 1000. An optimized scheme is proposed for reducing the bending of the grating. Through a dynamical experiment which shows the modulation effect of brightness and darkness.