Fault control strategies for the modular multilevel converter（MMC）based HVDC Flexible transmission system under unbalanced network conditions are described，and the advantages and shortages of various control designs are compared.A fault control method based on fault negative sequence voltage overlap is proposed according to the analysis and derivation of the MMC mathematical model.This method is simulated with off-line simulation tools and verified with tests in an actual project，and the results of the simulations and the engineering test are presented.The result comparison of simulations using different control strategies and the engineering test results show that the proposed control strategy is helpful for uninterrupted operation of the modular multilevel converter based HVDC Flexible system under system faults.