Abstract: Passive athermalizing technology of low-cost mirror-lens infrared optical system was studied, and the athermalization relationship was deduced and the condition expression for mirror-lens secondary imaging system athermalization was got. Defocusing aberration caused by thermal change could be regarded as a kind of primary aberration, and the Mangin mirror was introduced as secondary mirror to eliminate the thermalization aberration. The same as the normal system, four methods could be used to get an athermalization optical system: appropriate focal power distribution, optical material and housing material matching, lens surface shape changing and asphere surface introducing. A generalized optical model was built, and the results show that the MTF is more than 0.5 at 17 lp/mm in -45℃~+60℃ with less than 0.52℃ Narcissus.