The occurrence of DC-side bipolar short-circuit faults in flexible DC transmission systems is currently a serious problem. In order to achieve fault clearing, based on the half-bridge modular multilevel converter (half-bridge modular multilevel converter, HBSM-MMC) and the reverse blocking type insulated gate double transistor (reverse-blocking insulated gate double transistor, RB-IGBT), an improved reverse-blocking hybrid sub-module topology with fault blocking capability is designed (reverse-blocking hybrid submodule, RBHSM), referred to as RBHSM. Then, on this basis, the fault current blocking mechanism of RBHSM is analyzed, and the analytical expression of fault current is derived. In the fault blocking phase, all capacitors in the sub-module are connected into the loop, and 2UC back electromotive force (EMF) is always provided to achieve fault current blocking. Finally, taking the sub-module topology as an example, an RBHSM-MMC simulation model is built in PSCAD/EMTDC software to verify the correctness of the proposed sub-module topology. The simulation results show that the novel sub-module topology proposed in this paper can quickly block the fault current and provide 2UC back EMF in the fault blocking process. This work is supported by National Natural Science Foundation of China (No. 5170070473) and the Fundamental Research Funds for the Central Universities (No. 106112017CDJXY150101).