The influence of temperature on an infrared optical system is analyzed. The optical instruments for military use are usually used in a wide range of temperature. Therefore, to eliminate image plane defocusing, an effective temperature compensation measure must be taken. The athermal design approach and principle of an infrared optical system are presented. According to the design parameters of a small infrared optical system, an athermal design idea for passive optical compensation is proposed. The experimental result shows that the infrared optical system can achieve good imaging quality over the temperature range from 0℃ to 60 ℃.