Modular Multilevel Converters (MMC) has been applied in HVDC transmission system. The safe operation of the modular multilevel converter based HVDC system is inseparable from the reliable HVDC line protection. In this paper, through the series inductance on the HVDC line, the characteristics of the fault additional state of the different short-circuit faults in the DC line of the MMC-HVDC system are analyzed, and a protection strategy is obtained with voltage polarity on both sides of the series inductor. By analyzing the polarity of the voltage fault component of the positive and negative poles of the HVDC line, the fault pole can be judged. By analyzing the magnitude of the voltage fault component on both sides of the series inductance, internal or external fault can be distinguished. In the end, a simulation model of MMC-HVDC system is established in PSCAD. Based on the model, different faults type are simulated, and the protection algorithm is verified by Matlab. Simulation results showed that the proposed protection principle is correct and effective. This work is supported by Key support projects of the National Natural Science Foundation (U1766209) and National Natural Science Foundation of China (No. 51777166).