A strict express of signal sideband in the polarization coding subcarrier multiplexing quantum key distribution scheme is presented. The source of quantum bit error rate in the modulation process is analyzed. A filtering scheme based on double fiber Bragg grating (FBG) for the polarization coding subcarrier multiplexing quantum key distribution is presented. The reflection ends of these two FBG are placed toward each other. The reflection spectra are used to filter out the signal frequency from clutter frequency. The simulation results show that, the reflection structure can filter the intermodulation sideband except signal frequency and decrease the quantum bit error rate.