In order to describe principle and method of the athermalizing technique using simple me- chanical framework in this thesis, firstly, three ways of athermalizing of infrared optical system are com- pared.Secondly, an optical system working at 3～5 μm, 3°field of view and - 40～60 ℃ temperature range was designed. Furthermore, the image quality approaching the diffraction limit in a larger spatial field view and wider thermal range can be obtained when simple mechanical framework is used in the optical system.