Aiming at proposing improvement to the protection strategy of sub-module overvoltage bypass according to the condition that sub-module capacitors are charged in modular multilevel converter （MMC） blocking state， the phenomena that the voltage magnitude of sound phase is raised are analyzed when a grounding fault occurs at valve side and MMC is blocked in an MMC-HVDC symmetrical monopolar bipolar system. Because there are distributed capacitances in DC lines， the voltage of sound phase arm capacitance is raised by charging. The conditions and factors to cause and influence the charging process are presented. Then a new protection strategy to prevent mistake bypass of sub-module because of overvoltage in blocking state is proposed. Finally， simulation analysis is performed on a PSCAD model of a three-terminal MMC-HVDC system and the boosting mechanism of submodule charging in blocking state is fully verified. The proposed new protection strategy can prevent mistaken bypass of sub-module in blocking state and reduce maintenance workload of MMC.