Abstract: For the traditional infrared continuous zoom system difficult to meet the requirements of high zoom ratio and large relative aperture simultaneously, the structure of compound zoom optical system is used to increase the range of focal length and relative aperture of the traditional infrared continuous zoom optical system. Based on the long-wave uncooled focal plane detector with the pixel number of 320×240 and the pixel size of 25 μm×25 μm, a high zoom ratio long-wave infrared(LWIR) zoom optical system was designed. The optical system consists of a continuous zoom system part and a two-step zoom system part. The diffractive optical element is utilized to correct the chromatic aberration at the long focal length of the system. The working band of the system is 8 μm to 12 μm, the range of the focal length is -9 mm to -272.25 mm, and the F number is 1.4. The system has such advantages as good imaging quality, high zoom ratio, large relative aperture, small lead and smooth cam curves.