Abstract: In order to obtain distinct images for the different distances of the Cassegrain system, the secondary reflector must be adjusted slightly by revolving with a screw thread so as to change the focus of the Cassegrain system. However，the revolving of the secondary reflector can lead to the cyclical shaking image, and this kind of shaking is forbidden in many highprecision optical instruments. Aiming at this problem,the factor causing the periodic shaking of the image in the system was analyzed, and the specific mathematical formula was given distinctly.Additional, through a particular instance based on the precision of manufactory, the maximum deviation under this precision was analyzed,and the fine adjustment mechanism of secondary mirror without revolving the screw thread should be utilized if the shaking radius is beyond the range of allowable error. Results show that the fine adjustment mechanism of secondary mirror with revolving lead can lead to 0.65 mm radius shaking,while the other method without revolving lead can lead to 0.13 mm, implying the precision of the system can be greatly improved.