We assessed the problem of low mixing efficiency caused by unstable signal polarization because of a moving reflector in a laser communication terminal with a periscopic scanner. A real-time polarization compensation method based on rotating waveplates is presented, which keeps the receiving signal light polarization at 45° linear polarized and improves system mixing efficiency. A geometric model of the laser communication terminal was first established, its polarization transmission characteristics were analyzed by three-dimensional polarization tracks, and a system polarization transmission matrix was calculated. The relationship between scan angle and polarization of the output signal was simulated. The connections between a polarization-compensating λ/4 waveplate and the λ/2 waveplate rotation angle and scan angle were established. These findings will pave the way for real-time polarization control technology for coherent free-space laser communications.