Most underground gas storage in China encounters the problem of evaluating gas drainage. In order to evaluate the gas reservoir parameters, we propose a mathematical model for calculating the gas drainage radius. The model describes the gas seepage mechanism based on material balance, and lets us evaluate the gas drainage radius for any well pattern and also to calculate the optimal production parameters in a limited production time range. Experimental results have demonstrated the accuracy of the model. We have conducted studies to assess the nature of the dependence on the drainage radius on various reservoir parameters. The results have shown that the radius depends on multiple variables. For example, the radius increases as production time and the rock permeability increase. The field tests conducted have shown that the radius values calculated based on the mathematical model quite accurately match the actual parameters for existing underground gas storage at the Dagang field.