Abstract: Gas extraction solution design and the investigation of the extraction effect in this coal seam is directly influenced by effective gas drainage radius in borehole along the seam. This paper, firstly, defines the concept of gas drainage radius of influence and effective gas drainage radius,on the basis of this, discusses the gas flow theory model in the gas drainage course in this coal seam, flowed equation and boundary conditions in ideal conditions were decided. According to site for relevant gas basic parameter data, obtains the effective gas drainage radius at different drainage time on K₇ coal seam of Dashun Mine by using the matlab software. Combined the spot measured effective radius data through traditional relative gas pressure method, the differences and reasons of theoretical simulated calculation and traditional field measurement method are analyzed comparatively, furtherly the feasibility of this theoretical calculation are proved. This provides a simple and quick method for determining the effective gas drainage radius in this coal seam.