There are imbalance problems in the State of Charge (SOC) of batteries in grid-connected cascaded-type battery energy storage systems. Thus an improved droop control considering SOC balancing strategy is proposed. Based on traditional droop control, this method introduces a distribution correction term by considering SOC real-time estimation information of each battery pack. Then the reference active power and reference voltage amplitude can be reasonably distributed according to the SOC information, and the SOC balancing of each module is achieved without affecting the normal power output as well as keeping synchronization with the grid. In this way, the SOC balancing speed can be adjusted by changing the correction coefficient. In addition, stability analysis of the system based on singular perturbation theory is carried out, and the design basis for control parameters is given. The simulation results under different conditions verify the correctness and validity of the proposed control strategy and the theoretical analysis. This work is supported by the Natural Science Foundation of Hunan Province (No. 2017JJ2348).