Considering the problem that Half-Bridge Sub-modules which are widely used in the current flexible HVDC projects cannot deal with DC fault, a sub-module topology with DC fault current self-blocking capability is proposed. Firstly, the basic structure and operation principle of the proposed novel sub-module topology are expounded. Based on the equivalent circuit of the converter station during the time of DC fault, the fault current blocking mechanism and the voltage withstand level of the novel sub-module is analyzed in detail. Then calculate the hybrid implementation of the sub-module and compare the blocking capability and economic characteristics.Finally, a simulation model was built based on PSCAD/EMTDC software environment to verify the blocking ability of the proposed sub-module. From the simulation results, this sub-module topology can block DC fault current quickly and efficiently.