In order to achieve the aiming, identification and tracking of long-distance targets, photoelectric tracking systems often require their load systems to be continuous zoom, and have high requirements of large zoom ratio, wide band and consistency of optical axis. In this paper, the analysis of structural form and initial structural parameter of calculation of the continuous zoom system are introduced in detail, and on the basis of clarifying the relevant technical parameters, zoom ratio of 40, wide-band visible continuous zoom optical system is designed. The continuous zoom optical system is a designed to use spherical mirrors, which are easy to process, and the MTF value of each field of view is above 0.2 at 150 lp/mm, smooth cam curve, no inflection points, and good stray light control which has little impact on the system. Finally, through the location test, the system has good consistency of optical axis, which is less than 0.1mrad. Therefore, the performance indicators of the system are well met the requirements of the use of the equipment, providing reference value for the engineering implementation of the photoelectric tracking system.