The　energy　storage　units　of　modular　multilevel　converter　(MMC)　based　on　battery　energy　storage　system　(BESS)　are　dispersed,　which　leads　to　the　problem　of　state　of　charge　(SOC)　imbalance　between　energy　storage　units　during　steady-state　operation.　When　the　energy　storage　module　is　overcharged　or　over　discharged,　it　needs　to　be　out　of　operation,　which　will　affect　the　stability　of　the　whole　system.　This　paper　presents　a　SOC　equalization　control　strategy　for　submodule　battery.　By　changing　the　reference　value　of　DC　component　of　arm　current,　the　phase　to　phase　SOC　equalization　is　realized　without　changing　the　DC　side　transmission　power.　The　virtual　voltage　feedforward　is　used　to　realize　the　fast　equalization　of　SOC　between　arms　with　small　amount　of　calculation.　By　adding　a　power　scale　factor　to　the　submodule　switch　signal,　the　SOC　balance　of　the　submodule　in　the　arm　can　be　achieved　under　the　charging　and　discharging　conditions.　This　paper　deduces　the　power　transfer　relationship　of　MMC　based　on　BESS,　and　studies　the　equalization　control　strategy　of　battery　SOC　based　on　its　mathematical　model.　Finally,　the　effectiveness　of　the　control　strategy　is　verified　by　simulation.　The　results　show　that　the　SOC　balance　and　power　transmission　of　the　sub　module　energy　storage　unit　can　be　realized　under　the　three-level　control　strategy.　©　2020　IEEE.