The embodiment of the invention provides a method and a device for estimating the capacity of an air storage chamber of a compressed air energy storage system. Based on the operation state of the compressed air energy storage system, the aerodynamic principle of the air storage chamber and the thermal coupling effect of the surrounding rock of the air storage chamber, the air temperature change curve and the air pressure change curve of the air storage chamber are obtained. According to the air temperature curve, the air pressure curve and the second law of thermodynamics, the storage capacitycalculation model of the gas storage chamber is obtained. In accordance with storage capacity calculation model and a preset target ( storage capacity, the target capacity of the gas storage chamberis determined. Considering the aerodynamic principle of the gas storage chamber and the thermal coupling of the surrounding rock, Combined with the running state of compressed air energy storage system, the calculation of the storage capacity of the gas storage chamber is accurately modeled in order to achieve accurate evaluation of the capacity of the gas storage tank. The method can effectivelybalance the rapidity and accuracy of capacity estimation, can be used for early planning and design of compressed air energy storage system.