Four polarization states are used for key distribution in the BB84 protocol,but the polarization states will be rotated for magneto-optic effect during the transmission. Polarization variation will effect distribution of the key,so a means for polarization restoration is required. After the introduction of BB84 protocol and the four states the effect of polarization rotation on quantum key distribution is analyzed. The simulation by Matlab shows the rotation of polarization state will effect key distribution. At last,a feasible method is presented.