Energy storage (ES) system is a key component in new energy power system at present and in the future, application of energy storage system will promote the optimization of the grid structure and achieve friendly access and coordination control of new energy. The modular multilevel converter (MMC) is a member of multilevel converter family, and has been widely applied in the medium- or high-voltage high power applications. Progress of energy storage system based on MMC is concluded. Firstly, the operation principles and the technical characteristics of MMC are introduced briefly; secondly, the development of different energy storage technology is introduced. Then the energy storage integrating methods and the key technologies of MMC-ES such as modulation strategy, sub-model capacitor voltage balancing, main circuit parameter design and control strategy are summarized and analyzed. Finally, suggestions for key issues of MMC-ES are proposed.