Abstract: Passive athermalization technology of infrared zoom optical system is studied. Athermalization relationship was deduced and the condition expression for zoom system athermalization was obtained. Defocusing and chromatic aberration caused by thermal change can be taken as two kinds of primary aberrations, and can be eliminated with a passive athermalization system. There are four ways to implement athermalization in conventional systems, appropriate focal power arrangement, proper match of optical material, change of lens surface shape, and use of aspherical elements. A dual field of view (FOV) infrared zoom optical model was built, which had 6 lenses and one aspherical surface. It achieved good image quality in the range of -45℃～+55℃ in both FOV, with low Narcissus and less stringent tolerance requirement. Diffraction surface can be easily introduced in this model, which reduces the number of lens used and improve optical performance.