To equip a modular multilevel converter (MMC) with DC fault self-clearing and capacitor voltage balancing capability, a novel parallel dual-port submodule named clamp double full bridge submodule (CD-FBSM) is proposed. Both device cost and operating loss of CD-FBSM are low, and during a normal working state, there are multiple coordinated operation modes between adjacent submodules, and the balance degree of the capacitor voltage can be enhanced by the unique parallel mode. When CD-FBSMs are blocked because of a fault, capacitors inside submodules are parallel while capacitors between submodules are serial, and all capacitors are reversely connected into a circuit. The fault current can be reliably blocked and the voltage of capacitors is balanced. This is conducive to a quick system restart. In addition, a three-stage analyzing method for the fault current blocking mechanism is proposed, and the fault current blocking procedure of CD-FBSM is researched. Simulation results based on Matlab/Simulink show that the proposed submodule has high balance degree of capacitor voltage, and the fault current can be quickly blocked while the blocking procedure is consistent with the theoretical analysis. This work is supported by the National Natural Science Foundation of China (No. 52077079).