A set of long-wave infrared continuous zooming optical system was designed for airborne photoelectric detection equipment,based on long wavelength 320×256 element cooled thermal IR Focal Plane Array(FPA) detector.The optical system is made of two kinds of common infrared materials,Ge and ZnSe.By introducing a diffractive surface and three aspheric surfaces,the chromatic aberration and the off-axis aberration are well corrected, and the Modulation Transfer Function (MTF) of the system is kept above 0.35 for all continuous zooming ratios at the spatial frequency of 16 lp/mm.The system is consisted of 6 lenses,has a F/number of 3,working wave band of 7.7~10.3 μm,and cold shield efficiency of 100%.Continuous zooming from 50 mm to 400 mm is realized with smooth zoom paths.The results show that the compact structured continuous zooming system has pretty good imaging quality.