Abstract: Zoom optical systems with mechanical compensation are widely used in all kinds of optical instruments due to its unique advantages. The structures and principle of mechanical compensation zoom optical systems with 3-unit were briefly described. Cam mechanism and guide mechanism to improve the image quality of zoom optical system were discussed. Actual focal length output with changing focal length was analyzed, including implementation of focal length output and some problems. It is shown that proposed methods are feasible for improving the design and adjustment of zoom optical lens, in addition, the accuracy of actual focal length output at different focal length is improved significantly.