A large relative aperture athermalized infrared optical system with wide field of view is introduced. The working wavelength is 8-12 μm;the full field of view is 40°;the effective focal length is 6 mm;the relative aperture is 1.25;the total optical length is 50 mm;and the back focal length (BFL) is 15 mm. It consists of a diffraction surface and two aspheric surfaces with a triplet structure,only using germanium and ZnSe. The system is simple,low mass and high image quality. The evaluation of the system is given in the temperature range of -80 ℃-200 ℃. The results show that the modulation transformation function (MTF) is near to 0.7 at the spatial frequency of 16 lp/mm and also near to the diffraction limit.