The effective management and control of grid-connected DC micro-sources is the key to ensure the steady operation of DC microgrid. Droop control is a common method to manage and control DC micro-sources in DC microgrid. However, the traditional droop control based on State of Charge (SOC) has the defect that the voltage drop of DC bus becomes worse with the decrease of SOC. Therefore, an improved droop control based on SOC is proposed. Firstly, the law of adjusting the droop coefficient according to the voltage fluctuation of the bus is given, which will automatically reduce the droop coefficient when the bus voltage drops. Meanwhile, a system control model with voltage and current of output capacitance as state variables is established, and a double-loop PI controller with current inner loop voltage outer loop is designed. Finally, Matlab/Simulink simulation model is built, and the control performance of the system under the influence of four different factors is compared and simulated. The simulation results show that the improved droop control can achieve the voltage stability of the bus and the reasonable distribution of the power of the micro-sources according to its SOC, and it has a strong ability to resist the load change. This work is supported by National Natural Science Foundation of China (No. 61703145).