According to the requirements of the dynamic infrared scene simulator technology, a projection optical system of dualband infrared based on digital micromirror device was designed. The focal length of the projection system was 150 mm, the relative aperture was 1∶2 , the full field of view was 5.3° and the working wavelength covered the infrared middle wavelength (3~5 μm) and infrared long wavelength (8~12 μm).The system chose coaxial type and consisted of only three pieces of spherical lens and one projection prism. Ge, ZnS and ZnSe were used in the system. This system was simple, light weigh and low cost by introducing one binary surface. The designing results show: at the cutoff frequency 17 ly/mm and the image distance 5.6 mm, the modulation transfer function of middle wavelength is close to 0.7; the modulation transfer function of long wavelength is greater than 0.5; both are close to the diffraction limited curve. The system can meets the requirements of the overall design of the infrared scene simulator.