In order to obtain the true optical characteristics of a Digital Micro-mirror Device (DMD), a test method for the stray light distribution of the micro-mirror unit was proposed and an experimental device was built to test the stray light distribution of a micro-mirror unit in the 2×2 array area.First, a stray light test method is proposed. Then, in view of the small size of the micro-mirror unit and the flexible configuration mode, an illumination system with a continuously adjustable convergent spot size and an imaging system that can clearly image the micro-mirror unit was designed. Finally, the stray light distribution of the micro-mirror unit in the 2×2 array area was obtained through experimentation.The test results show that the reflection energy near the center channel of a single micro-mirror unit is strong, while the reflection energy near the edge is relatively weak. In addition, the micro-mirror unit also reflects part of the energy outside the test area. The maximum absolute stray light intensity of the micro-mirror unit in the test area appears near the central channel, and its gray value is 6.86. The maximum absolute stray light intensity of the micro-mirror unit close to the test area also appears near the central channel, and its gray value is 4.01, which indicates that the stray light near the central channel is strong. The relative intensity of stray light in the test area is relatively weak, which increases sharply from the edge of the test area and reaches the value of 293.5% after about two micro-mirror units, and then decreases sharply.