Abstract: The performance of the distance protection based on industrial frequency variation in an AC-DC hybrid power grid is discussed.When faults occur on the AC transmission lines close to the converter station at the receiving or sending end,due to the impact of control system,the DC system has characteristics of a weak power source and its equivalent impedance is significant.The operation range of distance protection is closely related to the ratios of system equivalent impedance and setting impedance,and the smaller of the ratios,the larger of the protection operating range.Therefore,the operating range of distance protection reduces greatly and it can lead to the under-reach and even operating failure of the protection.Finally,the correctness of theoretical analysis is verified by the real time digital system experiment data.