In view of frequent and serious DC side fault problems of MMC-HVDC, this paper analyzes the operating principle and working characteristics of modular multilevel converter (MMC), proposes that sub-module voltage equalization control and inter-phase circulation suppression can better fit for MMC DC transmission system, and designs the corresponding controllers. A simulation model is constructed in MATLAB/SIMULINK environment, then the simulation is conducted on MMC-HVDC DC side single-pole grounding, bipolar short circuit, and broken-line fault, and the simulation results show that the proposed control strategy can ensure the system to maintain stability after the failure, and improve the ability to solve the fault in MMC-HVDC DC transmission system. Finally, more suitable fault protection measures and over-current suppression methods are prospected, which have important guidance for solving DC side faults.