With the discrete phase encoding, a free space quantum key distribution system is constructed based on the phase synthetic polarization technique. On the basis of the temporal filtering, spatial filtering and spectral filtering, the polarization filtering combined with the model of the polarization degree of the atmospheric background light is used to suppress the background noise. The research results show that, the background light suppression ratio is up to 5 when the polarization degree of the atmospheric background light is 0.6. Compared with the situation without polarization filtering, the bit error rate of the system caused by the background light is reduced by 80%.