Abstract: A long focal length lightweight zoom optic system was designed with a focal length of 30 mm ~ 300 mm, an angle of view of 1.1 ° ~ 11.4 ° and an F number of 3.5. Due to the long focal length of the zoom system and the need to reduce the quality under the premise of control caliber, after the analysis of the zoom theory and the actual situation, the positive compensation was adopted, and the Zemax software was used to simultaneously optimize the image quality and design the lightweight zoom system. In the optimization process, the aspherical surface was added to simplify the structure and improve the image quality. Without affecting the illuminance of the image surface, the off-axis light was properly blocked, so that the effective aperture could be as small as possible, and at the same time part of the lens material was replaced, balancing the contradiction between high image quality and light weight, and finally the overall mass of the system reduced from 937 g to 584 g.Results show that the system quality is good, the on-axis modulation transfer function(MTF) is above 0.3 at 120 lp/mm, the off-axis MTF is greater than 0.2 at 120 lp/mm, the MTF for each field of view is greater than 0.5 at 40 lp/mm, and the distortion is less than 1%. According to the zoom motion equation, the programming calculation was done by Matlab software, and the cam curve reflecting the motion process of the zoom group and the compensation group was obtained. In the process of zooming, the image surface is relatively stable and the focusing is smooth.