Abstract: The digital micromirror device (DMD) as spatial modulation component is often used in projection optical system, when the incident light is long wave infrared, the diffraction effect affects the energy distribution into the system. The effect of the light incident angle on diffraction effect was mainly discussed. As a blazed grating model, the DMD was used to study the diffraction effect produced in long wave infrared band. Firstly, from the angle of optical path difference, the diffraction effect when the main and counter diagonals were in the open state was analyzed, and the diffraction effect was related to the incident angle. Then, the DMD was used as a blazed grating model and the vector diffraction theory was used to calculate the diffraction efficiency of DMD in the open state at 7.7 μm~9.5 μm band. The results show that when TM polarized light irradiating at an angle of 44°, the first order diffraction reaches a blazed state, and the diffraction efficiency can reach to 70%.