The traditional virtual synchronous units ignore the power support source and the dynamic security constraints of the DC grid. For this reason, a method of inertia response and primary frequency control of MMC-HVDC connected to passive power grid considering the energy buffer source is proposed. Firstly, the rotor motion equation of synchronous generator is embedded into the control loop of MMC, and the virtual synchronous control model of passive network inverter is built, and the evaluation index of frequency response is established; by comparing and analyzing the response of synchronous generator and virtual synchronous control MMC after frequency accident, the control loop of DC capacitance energy buffer is designed, and the charging and discharging process of capacitor responds to the dynamic frequency process of virtual synchronous machine; finally, the simulation results show that the method of proposed frequency stabilization improvement can increase the inertia and primary frequency modulation response capability of passive network under the premise of fast energy support of DC power grid.