Optomechanical systems are typically made of several materials with different thermal properties. These materials compose the optics (refractive or reflective elements) and the mechanics (optical mounts and system housing). As the temperature of these materials change, the volume and index of refraction will change as well, increasing strain and aberration content (primarily defocus).  Compensating for optical variations over a temperature range by combining, distributing focal power and selecting proper materials with reference to the differences between thermal properties of infrared optical materials is called passive athermalization. The compensation technology of passive athermalization can make the system have better temperature stability and more compact structure without move any parts or use all kinds of complex electronic equipment. The compensation design of passive athermalization mainly includes three processes: 1) Select appropriate optical materials according to the temperature index, thermal expansion coefficient and other parameters of infrared optical materials. The aim is to better correct the defocus of image plane caused by temperature change. 2) Select the proper combination. So that the material combination can meet the requirements of the system’s focal power while athermalization. 3) Under the premise that athermalization and focal power are satisfied, the design need to be further optimized to correct the aberration of the optical system.