The principle which can cause the defocus of the image surface is discussed. The domestic and foreign athermalisation methods are introduced, the advantages of passive athermal compensation of the optical system are analyzed, athermal equations of thermal compensation are derived. An athermal compensation method which combines the optics with the machinery is proposed. Using this method, the optical lens and the transfer function can keep relative stability, when the environment temperature changes, that is, the athermalization of optical system is basically realized, it is significant for athermal design of infrared optical system.