Diffraction effects play a significant role in DMD-based scene projection in the long-wave infrared band (8-12 μm). The contrast provided by these projector systems can become noticeably worse because of the diffraction effect of the DMD. So it is necessary to analyze the diffraction characteristics of DMD when it is applied to DMD-based scene projector in this wavelength band. Firstly, DMD was treated as a special blazed grating, and a grating diffraction model of DMD was constructed by diffraction theory and Fourier transform. Then, simulation calculation was conducted in MATLAB environment. Finally, the simulation result was verified by experiment of scene projection. It is shown that there exists a suitable illuminating incident angle range to achieve high contrast image in the long-wave infrared(8-12 μm) band. When incident azimuth angle is 0°and incident zenith angle is between 44°and 48°, the scene projector will have a good imaging contrast.