Based on the basic principle of geometrical optics, an analytical method is presented in this paper. Each optical surface is regarded as an infrared source, and all optical surfaces behind it make up a new optical system. The sampling rays carrying certain energy are used to indicate the thermal radiation beams, and they are traced through the optical system to obtain the energy distribution on the image plane. The influence of thermal radiation in the infrared optical system is further analyzed. An infrared optical system is designed, which has been athermalized. The thermal radiations from each optical surface are analyzed and the superposition result is obtained.