The invention discloses a fault-tolerance-capability-equipped MMC sub-module structure capable of realizing DC fault self-protection. Current stress balance of two capacitors and eight power switch tubes in a sub-module are adjusted through redundant switch states; three level can be output at a normal mode, and the level integration of the sub-module is improved; under the condition of a locking mode, the capacitors of the sub-module are completely invested into legs, the capacitors of the sub-module are charged, a reverse electromotive force is generated, and a function of isolating DC-side faults is realized. According to the invention, when the sub-module structure is applied to an MMC-HVDC system, self-protection of the DC-side faults can be realized. Due to the symmetry of the structure, the output characteristics of sub-module structure, under the condition of the locking mode, are symmetrical about a current direction, and good symmetry helps to maintain the current stress balance of the power devices and the capacitors in the sub-module; and when the MMC sub-module structure is applied to an HCMC-HVDC system, the MMC sub-module structure is lower in conduction loss than a full-bridge sub-module structure and higher in system operation efficiency.