The constant active power control mode of MMC-HVDC system is based on the power order sent by dispatching operators, and cannot adjust the transmission power according to the status of AC system. According to the characteristics of power transmission in AC lines, an asynchronous automatic control strategy for MMC-HVDC system is designed, which enables the MMC-HVDC system to automatically adjust the transmission power and realizes the function of simulating AC tie lines. The basic principles of asynchronous automatic control strategy, the detailed value-taking method of virtual impedance, and the scheme of function on/off are elaborated. Then, the simulation model based on actual AC power network is built on RTDS platform according to the control and protection devices of practical MMC-HVDC project. Finally, the effectiveness and feasibility of the proposed control strategy is verified.