According to the mathematical model and control strategy of half-bridge modular multilevel converter（MMC），a simulation model for multi-terminal HVDC based on MMC（MMC-MTDC）is built by use of PSCAD-EMTDC.A multilevel control system suitable for AC/DC power grids MMC-MTDC is designed including firing control，converter control and system control.In order to investigate the dynamic response characteristics of the AC/DC power grid with MMC-MTDC，AC grid faults，DC line faults and impulse load response are simulated.The simulation results show that the coordination between controllers at all levels makes the MMC-MTDC system ride through faults safely，and the whole AC/DC power grid remains stable when either end of MMC-MTDC exits.In addition，the AC power grid remains stable after the whole MMC-MTDC exits during DC line fault.