One of the key problems in the use of underground gas storage is frequent leakage. It can lead to the actual gas storage amount being less than that accounted for. Combining numerical simulation and parameter auto fit, this paper ascertains the dynamic variation of the pressure in the storage reservoir, adjusts the actual injecting and producing gas to fit the accounted pressure with the tested pressure, obtains the gas leakage of the storage, and then determines the difference between accounted amount and leakage amount. The result is the actual reserves of the storage. The simulation result shows that the method presented can provide a theoretic foundation for estimating the leakage amount, thereby ensuring the actual reserves, searching the leakage route,and reducing leakage by adjusting the storage method.