The changes of temperature may lead to the divergence of best focal plane of its optical system, which will make the imaging quality of optical system decline. In order to maintain the optical system imaging quality stable, we study the athermalization of the optical system. A new way of athermalization what is divergence of the best focal plane is studied. The surface displacement formula is studied, then the surface displacement formula is used to guide the design of athermalization compensation. For example, we study the system that reaches diffraction limit in normal temperature. When the temperature ranges from -40 ℃ to 60 ℃ , it is found that modify the four parameters in the surface displacement formula will make the surface displacement decrease or go to zero. So the optical system imaging quality gets improvement and achieves athermalization. The conclusion is that the surface displacement formula can guide the design of athermalization.