Abstract: In order to improve the optical energy utilization of the maskless lithography system based on digital micromirror device (DMD), the phase modulation properties of DMD at UV laser illumination were investigated. The impact on the phase modulation properties caused by the processing error of DMD was analyzed. The diffraction patterns of three DMDs of the same model at the UV laser illumination were tested, and results showed that their light field distributions were significantly different, the DMD processing error had great impact on the phase modulation properties. This conclusion has great reference significance for choosing DMDs with high utilization efficiency.