In this paper,a wide angle optical system for a cooled infrared dual band detector with passive thermal aberration is designed.The optical lens is composed of 6 lenses,and the athermal design is carried out by matching different optical materials to realize passive athermal design in the range of environment temperature -40～70℃.The optical system is designed with medium and long wave confocal plane.The designed wave band is 3.5～4.9μmand 7.5～9.5μm,the focal length is 11mm,the F number is 1.6,and the full field of view is 150°.The results show that the optical system has the advantages of large operating range,simple structure,and high transmittance and can achieve good imaging quality in the full operating temperature range.