The lidar polarization system realizes imaging for targets by the polarization differences of light scattered by the different targets. Because there exist the depolarization effects in a majority of systems, the depolarization problem of system itself must be considered in the process of imaging. According to the adaxial theory and requirement of optical aberration, the geometrical structure of Cassegrain telescope is designed. Then the depolarization behavior of the interface between air and reflecting mirrors of Cassegrain telescope is analyzed by using Jones formalism, coherent matrices of optical waves and law of Fresnel reflection. And the polarization degree of light is emulated, which is reflected from every point of reflecting mirror coated with and without thin film, relative to the diameter of Cassegrain telescope through the help of Matlab software. Based on the simulation, it is deduced that the polarization degree of light reflected from the telescope overcoated with aluminium film in the imaging system can be ignored in the imaging process.