As an important abnormal operation of coal mine's ventilation management, the design of gas drainage from a sealed lane in coal mine should be paid more attention to the safety of drainage process. On condition that the gas drainage rate is unchanged, the methane concentration during the gas drainage process would uniformly vary; the fresh air flow is able to be mixed sufficiently with those containing more methane in the lane. Based on the comparison with the traditional methods for gas drainage, a new mathematics model for gas drainage is established; the model deduces the dependent relation among parameters involving the methane concentration in drained lane, the flow rate of gas drainage, gas drainage time, and the flow rate under the total negative pressure ventilation system. Furthermore, the length of dangerous time and the risk during the gas drainage process are calculated in order to obtain an important conclusion, that is, the dangerous time of gas drainage is irrelevant to the initial concentration, the risk of gas drainage has noting to do with the ratio between the gas drainage flow rate to the volume of drained lane. Finally, a case study of gas drainage is adopted to validate the new proposed model.