Abstract: In order to realize the radiation detection of the full spectrum imaging spectrometer, a large aperture off-axis reflective solar simulator was designed, which can offer the natural sunlights irradiation under the laboratory conditions. The design of the optical system of the solar simulator was discussed in detail with the simulation and analysis by the Lighttools. According to the layout of the optical system and the thermal load distribution of the system, by using ICEPAK, the air cooling system was designed and tested. The results show that under the condition of satisfying the system cooling requirements,a 1.17 constant solar irradiation can be achieved by our solar simulator, the irradiation non uniformity in 200mm range is less than 1.61% and less than 3.28% in 400mm range.