The invention discloses an asymmetric operational control method of the direct current side monopolar grounding fault of an MMC-HVDC system. For the MMC-HVDC system based on monopolar symmetric wiring, after a direct current side monopolar grounding fault occurs, over voltage and fault current at AC and DC sides can be rapidly eliminated by setting the DC component of the output voltage of a bridge arm of a fault pole without locking a current converter, and further the insulation threat to AC/DC systems can be eliminated. Through adjusting phase angles of alternating components of different bridge arm voltages, the system still can continue transmitting half of a rated active power and provide reactive power support for the AC system while isolating the direct current side monopolar grounding fault, which has positive significance for the stability of the connected AC/DC systems; during the fault period, the current converter does not need to exit from operating, the restore speed of the system is high, the current converter is in a controlled state in the whole process, and therefore the active defense capability of the MMC-HVDC system based on monopolar symmetric wiring against the direct current side monopolar grounding fault can be improved.