The invention discloses a reduced voltage operation method for a hybrid MMC-HVDC (modular multilevel converter based high voltage direct current) system. Full-bridge sub-modules exist in a hybrid MMC, so that negative voltage can be output by bridge arms to achieve flexibility and controllability of the voltage of the alternating current side of the hybrid MMC. The sum of the output voltage of the upper and lower bridge arms is regulated to regulate the output voltage of the direct current side of the hybrid MMC. The difference between the output voltage of the upper and lower bridge arms is regulated to regulate the alternating current internal potential output by the hybrid MMC, namely regulating the output voltage of the alternating current side of the hybrid MMC to regulate active power transmitted by the hybrid MMC and reactive power exchanged with an alternating current power grid to ensure safe and reliable operation of an alternating and direct current system. Meanwhile, during reduced voltage operation, low-frequency loop current is injected to the bridge arms of the hybrid MMC, so that the problem of incapability of half-bridge sub-modules in voltage balancing when relatively higher active power is output under relatively lower direct current voltage is solved, the operation range of the hybrid MMC under low direct current voltage is extended, and more active power can be transmitted.