对比了典型消热差方法的优劣，探讨光学被动式消热差的基本理论。在此基础上，根据系统要求的温度范围-60℃～90℃，在常温初始结构的基础上，利用Zemax软件的多重结构和自动热分析功能增加其他温度结构，运用光学被动式消热差方法进行热平衡和像差平衡，最终设计出一套中波制冷型消热差光学系统。光学设计时以探测器冷阑作为系统孔径光阑，实现了100%冷阑匹配。结构材料使用铝，光学材料为硅、锗和硒化锌,将它们组合消热差。系统在-60℃～90℃温度范围内，最大离焦量小于1倍焦深，空间分辨率17 lp/mm处，光学调制传递函数(MTF)值均大于0.74，接近衍射极限，点列图弥散斑均未超出单像元尺寸范围。

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.