The invention discloses an energy-releasing generating device and method of a compressed air energy storage system. The device comprises an air storage unit, an expansion unit, a generating unit and acontrol unit. Information of the air storage unit, the expansion unit and the generating unit in a working state is collected in real time through the control unit, the rotation speed of an expansionmachine and the rotation speed of a generating machine are optimally controlled, and under the condition of ensuring that the output power is fixed, the highest energy efficiency is achieved with thelowest air consumption. The situation that a traditional compressed air energy storage system wastes compressed air is effectively avoided, and the quality of output electric energy is improved.