Abstract: The paper describes the state of CNG in gas storage well using BWRS equations, determines the density of CNG in gas storage wells with secant method, and establishes the leakage calculation model for underground gas storage well of CNG filling station based on the mass balance equations of CNG storage wells, and elaborates the calculation and monitoring steps, and developes leakage calculation and monitoring procedures based on VB6.0 and performes case study. The results show that the capacity of gas storage well is proportional to the pressure and in inversely proportional to the temperature, however the temperature of the underground gas storage well is close to the ground temperature, which is relatively constant, so the well pressure is the main factor that reflects the capacity of gas storage well, and if the leakage occurs, the pressure will change obviously. This calculation method has high accuracy and is relatively stable, so it can meet the application requirements of the general engineering, but its maximum error can not be ignored, and thus tiny leakage can not be accurately judged.