The invention provides a V2G-based constant-current discharge system which is capable of realizing the staging constant-current discharge of a storage battery for a power grid. The system is characterized by comprising a three-phase alternating current (AC) power supply connected with the power grid, a discharge module, a storage battery side voltage and current sampling module connected between the three-phase AC power supply and the storage battery for discharging, a three-phase AC power supply side voltage and current sampling module, a processing control module and an input setting module used for setting the strategy parameters of the processing control module, wherein the processing control module is used for outputting control signals according to the acquired voltage value and the current value of the storage battery as well as the set strategy parameters so as to control a lifting part to adjust the current value of the storage battery. After the V2G-based constant-current discharge system is adopted, the overlarge discharge depth of the storage battery is prevented, and rapid staging constant large current discharge of the storage battery can be realized, so that the problem that a pole plate is gradually vulcanized due to the continuous increment of discharge depth of the storage battery can be solved, the cycle service life of the storage battery is effectively prolonged, and the system stability is effectively improved.