Free-Space Quantum Communication(FSQC) based on the BB84(Bennett-Brassard 1984) protocol makes use of two groups of conjugate basis polarized photon as the carriers of information to realize the communication between a satelits and a group station. Polarization would show some problems like the contrast of polarization to be worser and the polarization-basis unmatched when transfering. In order to achieve the BB84 protocol, ground station needed to track and calibrate for the polarization zero direction of the satellite equipments. So the polarization maintaining and polarization-basis calibrating is one of the key technologies of the FSQC system. Firstly, this paper established the mathematical model of polarization in the FSQC system, and then derived the mathematical relation between the contrast of polarization and the phase delay in theory. Some proposals of Optical System designs have been suggested in the paper. Finally the problem of dynamic polarization-basis in the satellite-ground link had been analysed and the dynamic characteristic of the deviation angle of polarization-basis in this paper.