With miniaturization of the UAV of optoelectronic pod, the requirements of miniaturization and rapidity for its zoom system is raised. A small segmented zoom system is designed. The focusing components include stepper motor and permeability type of photo interrupter, and the hardware circuit of system and focus method is designed, which achieves the rapid segmented zoom of the optical lens and the real-time output of focal-length. The focal point position and focal length of system are calibrated by off-line methods. Tenengrad function, Brenner function and square gradient function are utilized to evaluate comprehensively during the calibration process of the focal point position. At the same time，it adopt auto collimation measured focus instrument to calibrate accurately the focal length of lens. Finally, it is achieved that this zoom system weighs less than 100 grams, the errors of focal-length is less than 1%, and the switching time between short focal and long focal is less than 1.7 seconds.