Energy storage unit is one of the key factorswhichcan ensure the reliable operation of DC microgrids. Traditional bi-directional Buck-Boost convertershave no advantages in voltage gain regulation and the correspondingcost, so a new design scheme is required to make a trade-off between the regulation capability and cost. First, for the energy storage system based on cascaded Buck-Boostconverter, this paper proposesa system-level design scheme whichusing cascaded Buck-Boostconverter as the sub module. Then, the voltage gain regulation of the cascaded Buck-Boostconverter in Buck and Boost mode is investigated by analyzing the working mode, and the modulation strategy is proposedfor the cascaded Buck-Boostconverter. At the same time, with the consideration of the operation mode of DC microgrid, the overall operation strategiesof energy storage system arefurther proposed to enable the storage to adoptthe appropriate control strategy to securethe reliable operation of microgrid under different working conditions. Finally, the effectiveness of the designed energy storage system and the control strategy areverified by several simulations.