To ensure a proper temperature rise of a Digital Micro-mirror Device (DMD), the thermal design of DMD was studied by calculation and experiment. A forced liquid-cooled thermal design was presented, and the temperature rise of each measurement point on DMD was calculated. Calculation results show that the maximum temperature rise of the DMD reflecting red light is 14.4℃, the maximum temperature rise of the DMD reflecting green light is 11.3 ℃, and the maximum temperature rise of the DMD reflecting blue light is 10.2 ℃, which have met the working temperature requirements. Thermal design and calculation are also verified through a thermal test and the maximum deviation between thermal calculation and thermal test are not exceeding 2.5 ℃. The thermal tests verify the reliability of thermal design and the correctness of thermal calculation, and supply a theoretical warrant for thermal design optimizing of DLP digital cinema projector.