The invention relates to the technical field of energy storage, and discloses a compressed air energy storage system and an energy storage method. The compressed air energy storage system comprises anair compressor, a heat exchanger, a gas storage, an air expansion, a heat regenerator and a heat accumulator. An air outlet of the air compressor communicates with a gas-phase space of the gas storage through a first heat exchanging pipeline of the heat exchanger, the gas-phase space is further connected with an inlet of the air expansion through a third heat exchanging pipeline of the heat regenerator, a heat storage medium space of the gas storage is connected with an inlet of the heat accumulator through a second heat exchanging pipeline of the heat exchanger, and an outlet of the heat accumulator is connected to the heat storage medium space through a fourth heat exchanging pipeline of the heat regenerator. According to the energy storage method of the energy storage system, energy-releasing storage is combined with a heat energy storage system, the gas storage, the heat exchanger, the heat accumulator and the heat regenerator are used for forming a heat regenerative cycle, internal heat balance and pressure balance of the compressed air energy storage system can be simultaneously realized, and the conditions of variable backpressure operation of the air compressor becoming and sliding pressure operation of the air expansion are avoided.