The common technical measures of optical passive athermalization are summarized.Aiming at the characteristics of uncooled long wavelength optical system,including large relative aperture,high transmission,small dimension and good environmental adaptability etc.,an optical system including three lens is designed and optimized by CODE V based on uncooled detector of 640×512 large focal plane,and the performance parameters of optical system include focal length of 70 mm,field of view of 10.44°×8.36°,F number of 0.85 and total length of 85 mm.In the range of -50~70 ℃,the modulation transform function(MTF) of all fields of view is greater than 0.61 at 25 lp/mm,and the distortion is less than 0.5%.The results of tolerance analysis show that optical elements processing and optical mechanical assembly are mature and stable,and optical system design has good engineering performance.