Abstract: The effect of temperature variation on infrared optical system and athermalization technique were introduced. An athermal middle infrared refractive optical system was designed by CODE-V optical design software，which used Ge, Al and ZnSe material. The image quality of the system approached diffraction limit in the temperature range of -40℃～65℃. The exit pupil and stop of the optical system was coincided. The optical system has many advantages，such as simple structure，small volume，light weight and low cost．It can be applied to space infrared optical system.