Distributed energy storage systems (DESSs) play an important role in maintaining voltage stability in DC microgrids. In order to improve the inertia of DC microgrid and balance the charge/discharge power and the state-of-charge (SOC) of each energy storage unit (ESU), an SOC-based virtual DC machine (VDCM) control strategy for DESSs in DC microgrid is proposed in this paper. By adding SOC to the armature resistance, the improved virtual machine algorithm can dynamically balance power and SOC simultaneously. In this article, the transient stability of DC bus voltage is enhanced using the output inertia of VDCM, and the SOC differences between ESUs are rapidly reduced by introducing the average SOC of ESUs. Small-signal models of both each ESU-interfacing converter and DESS-inclusive DC microgrid with proposed control method are established. The impact of VDCM parameters on voltage stability is analyzed. Finally, a test DC microgrid is built to verify feasibility and effectiveness of the VDCM control strategy.