Abstract: In order to provide a simple and rapid method to solve the effective drainage radius, based on a gas radial unstable seepage equation proposed by Acad emician Zhou Shining, a self simulation method was applied to get the approximate solution, which would be the gas pressure distribution expression of the radial flow f ield. Thus a mathematical expression of the effective drainage radius was further established. The study showed that under the condition of an infinite flow field, the squ are root of the effective drainage radius and the drainage time would be in a direct ratio and the square root of the drainage negative pressure could meet the nonlinear positive correlation. Under the different drainage negative pressure condition, the gas pressure distribution law around the borehole would be small in difference. Theref ore, a simplified gas pressure expression would be applied to approximately reflect the gas pressure distribution law of the radial flow field. The approximate solution an d the effective drainage radius expression could be applied to determine the relevant parameters of the seam gas drainage. The related experiments were verified the a ccuracy of the effective drainage radius expression.