The utility model discloses an on-line active inverter modular circuit for a transformer station storage battery. The on-line active inverter modular circuit comprises an active inverter, a direct current contactor, and two active inverter alternating current contactors. The direct current contactor is connected with a direct current input positive end of the active inverter. The two active inverter alternating current contactors are connected with two alternating current output ends of the active inverter respectively. With the on-line active inverter modular circuit, the discharge process is enabled to be safe, energy-saving and environment-friendly; a storage battery cannot be disengaged from the bus in the discharge process, direct current discharged by the storage battery is converted into alternating current with the frequency and phase identical to that of commercial power, and then the alternating current is fed to an electrical network; and when commercial power or a charging device fails in the storage battery discharge process, discharge of the storage battery is immediately stopped and power supply for the bus is activated.