Abstract: To determine the effective gas extraction radius of the dill hole along coal seam reasonably, based on Darcy’s law, Fick’s law of diffusion, as well as gas flow method in the coal, gas flow equation was established, different determination methods were analyzed according to different gas extraction evaluation index, extraction rate was adopted to calculate the effective radius on the basis of Jingfu mine field gas attenuation coefficient, the results show that the effective radius increases to the limit of 1.4m gradually ,and the process slows down with time going on, at the time of 86ds the radius become 1.33m.