The invention discloses a gas expander acting system and method in a compressed air energy storage system. The gas expander acting system in the compressed air energy storage system comprises a high-pressure gas source, a steam source, a mixer, a gas sprayer and a gas expander; the gas sprayer is provide with an inner cavity, and a first inlet, a second inlet and an outlet communicating with the inner cavity; the high-pressure gas source and the steam source both communicate with the first inlet by the mixer; an exhaust gas outlet of the gas expander communicates with the second inlet; and anintake port of the gas expander communicates with the outlet. A high-pressure gas medium output by the high-pressure gas source is mixed with high-temperature steam output by the steam source to serveas high-pressure working gas currents to input in the gas sprayer; and as the high-pressure working gas currents mixed with the high-temperature steam are higher in temperature and enthalpy, the absorbing capacity on low-pressure exhaust gas can be effectively improved, the ejection coefficient and the working performance of the gas sprayer are improved, and the acting efficiency is improved.