According to the steady state mathematical model of modular multilevel converter（MMC），a discrete mathematical model of MMC is developed.Based on which，a discrete inner-loop current controller of MMC is designed.The positive-negative sequence passive controller based on an Euler-Lagrange mathematical model of MMC is proposed to suppress negative sequence current due to DC transmission system faults.In order to maintain certain active power delivery，the active power control strategy under fault condition is adopted.Finally，a21-level system simulation model of MMC high voltage direct current（MMC-HVDC）transmission system is built in PSCAD/EMTDC.The results show that the proposed control method has a good control effect on the transient and steady state process and can provide reference to actual projects for its simple structure.