Abstract: Based on the principles and methods of pressure index, the drilling drainage radius in a coal seam of Hebei is measured with the method, and the fitting curve between the effective drainage radius and drainage time is educed. The result shows that the limit effective influence radius of φ89 mm drainage drilling is 8.7 m in this coal seam, the limit drainage time is 300 d. Determining 90 d to the just drainage time, in other words determining 4.5 m to the just drainage radius is more economical and rational.