The invention discloses an LCC-MMC hybrid three-terminal high-voltage direct current transmission system and a DC fault fast current limiting method thereof. The LCC-MMC hybrid three-terminal high-voltage direct current transmission system comprises a transmitting end converter station LCC and receiving end converter stations, namely, two hybrid MMCs, and redesigns a control architecture of the existing hybrid direct current transmission system. The DC fault fast current limiting method provides a fault current limiting control strategy for the LCC and the MMCs, suppresses the magnitude of fault currents of the transmitting end LCC and the receiving end MMCs by adopting an additional control strategy during the system DC fault transient process, quickly suppresses the DC fault current whena DC fault occurs in the system, and protects current converter equipment. The LCC-MMC hybrid three-terminal high-voltage direct current transmission system and the DC fault fast current limiting method thereof realize the independent control of AC-DC control through AC-DC decoupling, enable each bridge arm to output a bridge arm output voltage reference value, and realize the control of AC and DC currents at the same time; and when a direct current fault occurs, the MMCs operate near a zero direct voltage through quickly switching the transmitting end LCC into an inverting state and negatively inputting full-bridge sub-modules of the receiving end hybrid MMCs, thereby realizing elimination of the DC fault current.