Abstract: Projection optical system is the key component of the infrared scene simulation system based on DMD. The projection optical collimating system was designed. The initial structure of the system was obtained by use of athermal optical design method for the complementary of different thermal properties. The one side was designed to the dual surfaces, the aberration was reducing, and the optical transmission properties was improved, system athermalized designs was achieved. The refractive structure was used by projection light path, working in 8-12 m long-wave infrared band, with focal length of 113 mm, half-field angle of 4.5 and F number of 1.3. Optical system using the software ZEMAX was optimized to meet the requirements for use of the simulation system.