With the application of DC microgrids more and more widely, the coordinated control study among multiple microgrids is significant. This paper proposes a microgrid cluster topology and based on it a coordinated control method is studied, which could realize the optimized management of power flow among multi-microgrids through the piecewise adjustment of common bus voltage. This strategy analyzes the power distribution inside DC microgrids and divides the operation states into three different modes, where different mode has diverse external characteristics. According to the external characteristics, the piecewise-droop control strategy of the grid-connected converters is designed, which can realize the coordinated control of energy and improve the utilization of renewable energy. The correctness and effectiveness of the proposed method are verified by simulation results.