Abstract: Aiming at the problem that diffraction effect of digital micro-mirror device (DMD) has serious influence on system imaging, the stray light generated by DMD device was studied. Firstly, based on the rigorous coupled wave theory, the DMD diffraction model was constructed and the effects of different bands and incident angles on its diffraction efficiency were analyzed. It was concluded that the diffraction effect of DMD was more obvious in the long-wave infrared band, and the diffraction effect of DMD was more significant with the increase of incident angle. Secondly, the generation methods of diffracted stray light of DMD in the projection system was analyzed. By changing the number of F in the system, the incident angle was controlled, and then the energy proportion of stray light generated by the diffraction effect of DMD was calculated. Finally, the curve of the optical system with different number of F at different wavelengths along with the diffraction effect was obtained. The results show that the larger the F number, the smaller the influence of DMD diffraction effect on stray light.