Aiming at the problem of low mixing efficiency caused by unstable signal polarization states because of moving reflector in the periscopic laser communication terminal, a real-time polarization compensation method based on rotating waveplates is presented, by which 45° linear-polarized receiving signal light is kept so as to improve the system mixing efficiency. The geometric model of periscopic scanner is established, and based on it, the polarization transmission characteristics are analyzed by three-dimensional polarization tracks method, and the system polarization transmission matrix is obtained as well. The polarization states of output beams under different azimuths and pitch angles are given and the connection between rotation angles of polarization compensation elements (1/4 waveplate and 1/2 waveplate) and azimuths and pitch angles of periscopic scanner is established. The effect of the waveplates rotating anlgle errors on the polarization state of output light is analyzed.