Digital micromirror device (DMD) is the key device of laser direct writing lithography intrusments (LDI). We establish a diffraction model of DMD based on LDI in production conditions,and analyse the relationship between the distribution of diffraction spot in the fourier plane and the image contrast in the image plane. The different performance of the image contrast using the same type DMD devices is theoretically explained, which is consistent with the exposure experiments. Additionally, we modulate the distribution of diffraction spot by adjusting the angle of the incident light and the performance of the exposure result is significantly improved. The poor image contrast of DMD caused by production batch is overcomed and the unusable DMD chips can be properly used. Thus the material cost of production is reduced.