Abstract: In order to improve the accuracy of automatic focal length measurement on the optical bench, a continuous zoom optical system was designed. The system is composed of selfcollimation component and zoom component, and can adjust the size of image on CCD target surface till the best measurement is got. According to the technical requirements of the zoom system and based on the total control, the initial structure selection and image quality optimization were done. After the completion of the design,the focal length of self collimation component is 200 mm,the zoom component is 200 mm～20 mm and the zoom system after docking can realize the image size of 1～10 of continuous adjustment. Simulation results show that the zoom system shows good image quality,the modulation transfer function (MTF) curve is close to the diffraction limit, and the dispersion spot is less than the size of CCD pixel. The above implies that actual testing requirements can be met.