Abstract: The pros and cons of the typical methods of athermal design were compared, and the basic theory of optical passive athermalization was discussed. According to the system requirement temperature range of -60℃ ~ 90℃, based on initial structure in normal temperature, using multi-configuration and automatic thermal analysis function, other temperature structures were added in Zemax software, then thermal and aberration were optimized. Finally a cooled medium wave infrared optical system was designed by using the optical passive athermalization. Detector cold-shield stop was used as system aperture stop, so this optical system could realize a 100% cold-shield-match. Optical materials were Si, Ge and ZnSe, and structure material was Al in the optical system. The MTF value is greater than 0.74, which is close to diffraction limit at the spatial resolution of 17 lp/mm, and the RMS spot diameter is less than single pixel size within -60℃～90℃. Those results show that the optical system has good image quality and athermal effect.