Grounding electrode is the most important part of HVDC systems. In order to improve the reliability of grounding electrodes line protection, a new unbalance current protection based on the longitudinal differential protection principle is proposed. The protection transformers are installed in the head and end of the grounding electrode line, the longitudinal differential protection scheme and criterion are presented, the original action strategy is optimized and the protection action characteristics are analyzed. The simulation results show that the unbalance protection is effective, which could accurately distinguish the fault types and act to isolate faults, and reduce the probability of HVDC shutdown.