To the satellite-to-ground (STG) optical communication, the circular polarization laser is an optimum choice for the information transmission. Firstly, the influence factors of the polarization characteristics of the optical system are analyzed by the Fresnel formula and the Jones Matrix. Then the structure of a practical STG laser communication transmitting optical system and the designed transmittance curves of several different bandpass filtering films in the system are given. Propagation of the linear polarization laser in the optical system is simulated by the optical software ZEMAX. The results indicate that the beam splitters and mirrors on a 45° tilt make the greatest contributions to the polarization state transform. Ordinary depolarization film can only prove that the power pcomponent and s-component transmittances are identical. But the phase delay of p-light and s-light still exist, which can make the polarization state change. By adjusting the direction of the incident light vector to the incident plane, the instrument polarization error induced by the optical system can be reduced to the minimum.