Abstract: Based on the theory of optical passive athermalization and the research of athermalization for dual-field-view un-cooled LWIR optical system, a LWIR optical system with large relative aperture and 3x step zoom was designed using chalcogenide glasses which posses good thermal stability and unique LWIR optical properties. The specifications are as follows: F/# is 1, EFL is 50/150 mm, and total length is 240 mm. The 640480 FPA detector was adopted whose pixel size is 17 m17 m with working band from 8 to 12 m. Three LWIR material was used in the system which were Ge, ZnS, IRG201. The image quality is diffraction limited from-40 to 60℃at both WFOV and NFOV.