In order to reduce the voltage fluctuation of the DC bus and enhance the immunity and inertia of the DC microgrid, an improved Virtual DC Machine (VDCM) control strategy is proposed for the bidirectional DC/DC converter (BDDC) on the energy storage side. By analogy with the converter current distribution equation and the DC motor rotor motion equation, the DC machine torque equation is embedded into the converter current distribution equation. Secondly, in view of the problem that the traditional U-I sag control can not eliminate the steady-state error of the bus voltage when the load power fluctuates on the DC side, the integration link is introduced in the sag control to prevent the bus voltage drop. A simulation model was built in MATLAB/Simulink to verify that the proposed strategy can effectively improve the inertia of DC microgrid and reduce the steady-state error of bus voltage.