Abstract: In order to eliminate the ambient temperature effect on the performance of infrared optical system in missile, an athermal infrared Cassegrain optical system was designed for 3.7 m~4.8 m band. The ANSYS software was used to analyze the effect of heated dome deformation on imaging quality of infrared system. Optical passive athermalisation technique was a proper choice, which could achieve the compensation for ambient temperature in the range of -40 ℃~+65 ℃. The results showed that the off-axis of MTF was greater than 0.41 at 20 lp/mm, the dispersion spot diameter was less than one pixel in different views and the structure of optical system was simple. This design ensured the stability of the optical system imaging plane and effectively improved the ability of detecting and recognizing targets for fire control system.