As a key component of the autonomous DC microgrid, the Energy Storage System (ESS) is mainly composed of multiple Energy Storage Units (ESUs). There is a problem in that the State-Of-Charge (SOC) equalization speed of multiple ESUs is slow, and a large bus voltage deviation will be generated during the SOC equalization process. Here we propose an improved SOC droop control strategy. First, the control strategy determines the optimal droop curve according to the charging and discharging state of each Energy Storage Unit (ESU) and the SOC value, distributes the load power reasonably, and reduces the bus voltage deviation. Then, by determining the main energy storage unit for power redistribution, and dynamically adjusting the droop coefficient within the allowable range, the system quickly converges to an equilibrium state, and the bus voltage deviation generated by this process is reduced. In addition, when the ESS is out of operation, the photovoltaic system stabilizes bus voltage, and the photovoltaic system is switched from variable step MPPT control to droop control with feedforward compensation to ensure bus voltage stability and safe operation of the microgrid. Finally, the simulation results in Matlab software indicate that the proposed control strategy can reduce the bus voltage deviation and maintain the stable operation of the autonomous DC microgrid while ensuring the rapid equilibrium of the SOC. This work is supported by the National Natural Science Foundation of China (No. 51607031) and the Science and Technology Innovation and Development Program of Jilin City (No. 20190104138).