An innovative converter bridge arm protection method is proposed for the modular multilevel converter - high voltage direct current （MMC-HVDC） system. Firstly, the current flow paths are analyzed when the single bridge arm of the converter is grounded and when the single-phase bridge arm is indirectly grounded. An equivalent circuit is constructed, and the expressions of the bridge arm current before and after the converter is locked are derived. Based on this, the unique characteristics of the sum of the three-phase bridge arm currents after a fault are analyzed, and a protection strategy is proposed to distinguish internal faults from external faults of the bridge arm by detecting whether the fundamental frequency components of the three-phase bridge arm and the current are zero. Given that the existing bridge arm protection methods cannot effectively deal with the short-circuit problem between single-phase bridge arms of the converter, the characteristics under such short-circuit conditions are further studied. It is found that the currents of the upper and lower bridge arms of the faulty phase only contain the fundamental frequency component and don’t contain the DC component, and thus the corresponding action criteria are formulated. By outputting the short-circuit protection criterion between the three-phase bridge arm and current protection and the single-phase bridge arm through or gate, various faults occurring on the converter bridge arm can be comprehensively identified, enriching and improving the protection principle of the converter bridge arm. Finally, by building the simulation model of the bipolar MMC-HVDC system in MATLAB/Simulink, the effectiveness and reliability of the proposed protection method are verified.