The influence of temperature changes on infrared optical system structure parameters was analyzed.The defocusing distance and modulation transfer function (MTF)caused by temperature shift were calculated.The theoretical basis of athermal design for infrared optical system was given.Based on a factual long-wave infrared optical system,the imaging qualities at the working temperature of 20 ℃,-40 ℃ and 60 ℃ were analyzed by ZEMAX.The analytic results show that the image quality of the system approaches the diffraction limit at the working temperature of 20 ℃.The MTF is greater than 0.6 in the whole field of view when the Nyquist frequency of detector is 20 lp/mm,and 87.6% of the diffraction energy is focused in a single pixel,and the imaging quality of the system is good.However,the imaging quality decreases seriously at the working temperature of-40 ℃ to 60 ℃,so the system can’t satisfy the design requirement.Based on the above analysis,the infrared optical system was redesigned by optical passive athermal with hybrid refractive/diffractive.The imaging quality of this infrared optical system is greatly improved after athermal design.The MTF is greater than 0.55 in the whole field of view when the Nyquist frequency of detector is 20 lp/mm,and the energy distribution is very concentrated,which can meet the requirements of infrared detection system.