To improve the spatial light modulation properties of digital micromirror device (DMD), and exert its performance in optical information processing, the phase modulation properties of digital micromirror device have been investigated. Based on digital micromirror device microstructure, its grating properties is investigated, its effect of phase modulation on coherent light is calculated, and the mathematic discription and experimental results are presented. The diffraction grating properties digital micromirror devices are mainly investigated, analyzes its working principle and the change of position between micromirrors by micromirror rotation is analyzed. With the theory of blazed diffraction grating, the phase difference between rotated mirrors has been calculated, the blazed rule of DMD gratings is found and blazed condition is realized. Methods of analyzing digital micromirror device phase modulation by mirrors on different directions have been proposed. The microstructure and working principle of digital micromirror device are deduced and simulated. Results on digital micromirror device light modulation are presented, which are meaningful for broad application of digital micromirror device in information optics.