A dual-band infrared optical system is designed for the dual-band cryogenic detector of 640×512 array, which can be used in the airborne infrared search and tracking system. The optical system adopts the combination of Ge, Zns, and ZnSe to achieve the achromatic and athermalization design.By introducing aspheric surface and configuration optimization, the high order aberration of the system is well corrected and the system structure is simplified.The optical system consists of only 8 lenseswith the working bands of 3.7~4.8 μm and 8~9.4 μm, F-number of 2, the focal length of 360 mm, and the FOV of 2.54°×2.03°, meeting the 100% efficiency of a cold aperture. The image quality evaluation results show that the imaging quality of the optical system is good in the temperature range of -55 ℃ to +70 ℃. The analysis of non-uniform temperature field shows that the optical system has a fine temperature gradient tolerance.