Abstract: Aiming at the athermal design requirements of infrared imaging systems for defense application such as simple structure, small size and light weight, this paper proposed an infrared optical system (8 m~12 m) with passive optical athermalization design. The parameters of the optical system was F=1，f=60 mm and 2=11.4.The analysis of results showed that the modulation transfer function (MTF) approached to the diffraction limit, the MTF was up to 0.7 in the central field of view(FOV) and better than 0.6 in the marginal FOV at the spatial frequency of 20 lp/mm in the temperature range of -40℃to +60℃. The results meet the athermalization requirements of the system.