Overcurrent caused by DC pole-to-pole fault in modular multilevel converter HVDC(MMC-HVDC) threatens system security seriously. The hybrid HVDC breaker is installed on the DC line to block the fault current. The analysis of fault mechanism is divided into the periods before and after converter is blocked. The overcurrent characteristics of bridge legs and DC line is emphatically analyzed caused by fault before the converter blocked, and time sequence control strategy of hybrid HVDC breaker is designed based on current characteristics. The MMC-HVDC and hybrid HVDC breaker models are built in PSCAD, the analysis of fault proposed is proved. Finally, simulation results verify the strategies proposed can effectively block the fault current.