More and more attentions are paid to the protection strategy for the MMC-HVDC. Due to the difference between the mmchvdc and traditional HVDC, a new flexible DC line protection scheme is proposed based on the boundary current. Firstly, transient characteristics of both ends of current waveforms are analyzed when various types of faults in HVDC lines occur in different positions. It is noticed that the line current in the rectifier is significantly greater than that in the inverter side during the transient process of internal fault. Comparably, there are no significant differences between the two ends of current value at external faults. Secondly, a DC line protection strategy for MMC-HVDC is proposed based on boundary current distance. Finally, the simulation and verification of protection strategy are carried out on the 21level MMC-HVDC system. It is shown that the proposed protection strategy can correctly distinguish the internal fault from the external fault, thus has high reliability.