The video microscope has been widely used in many applications with the development of CCD detector. The working distance of the video microscope is short due to the limitation of the catadioptric configuration and it affects its application. In this paper, we focus on the analysis of aberration characteristics which is influenced by the long working distance. At last, we use optical design software Zemax to simulate the optical effectiveness and give design results of cassegrain system where the working distance is 84.5 mm, the numerical aperture is 0.16, the optical magnification is 5, the field number is 1.2 mm and the resolution is 7.7 μm.