Based on uncooled detector,an optical system is designed with 100 mm focal length,1 F number,±2.5°field of view ,working in the 3-5 μm wave band. The optical system has the advantages of large relative aperture and compact construction. While working under the condition of -20 ℃-60 ℃ the system can keep high quality image. With the Petzval objective,carefully selected optical material and reasonable optical power distribution,passive optical athermalization can be realized by using this system. The hybrid lens is used to simplify construction and decrease the volume and weight of the optical system. Also,large back focal length is another feature of the optical system.