Isolating DC bipolar short-circuit fault is one of the difficulties faced by the flexible DC distribution system. This study aims to the hybrid modular multi-level converter (MMC). Firstly, the fault-blocking mechanism of a hybrid MMC is analysed. Through theoretical calculation, the influence of the proportion of full-bridge sub-modules η on the fault isolation capability is obtained in the process of bipolar short-circuiting of the hybrid MMC-isolated DC side. Finally, the hybrid MMC DC-side bipolar short-circuit fault is simulated and analysed by the simulation model built in PSCAD/EMTDC software. The fault blocking capability of hybrid MMC and the relationship between η and the full-bridge sub-module capacitance voltage after the fault isolation phase are verified, which provides a theoretical basis for the design of the withstand voltage of hybrid MMC full-bridge sub-module capacitors.