Abstract: In order to solve the problem of low sensitivity of protection when there are high resistance and remote ground faults of ground electrode lines in HVDC transmission system,a new principle of HVDC ground electrode line protection based on characteristic harmonic measured impedance（CHMI）was proposed. First, based on the equivalent circuit of the ground electrode lines, the CHMI of ground electrode line was defined, which is the ratio of characteristic harmonic voltage to the characteristic harmonic current of one line from the measuring end. And then the characteristics of the CHMI pre and post line fault were analyzed and simulated. Furthermore, a fault protection criterion based on the ratio of CHMIs’ amplitudes was constructed. Finally, in order to verify the performance of the proposed protection principle, the simulation results based on PSCAD/EMTDC show that the electrode line protection principle proposed in this paper can reliably identify the faults of almost the whole range of ground electrode lines with high transient resistance and noise interference. In addition, there is no need to install additional hardware. Therefore, it has good applicability and practical value in power system.