A measurement study was conducted for the effective drainage radius of borehole along coal seam #9 of the Kaiyuan Coal Mine using the gas pressure method and gas flow method. The measurement results show that the effective drainage radius of borehole along coal seam #9 was 0.75 m, 27 days after drainage, and 1.5 m, 92 days after drainage. Experimental schemes were designed for the entire evolution of the stress in the coal mass around the borehole, and an experimental study on methane seepage in the coal mass around borehole was conducted. Fitting functions for the permeability of the coal sample and its vertical stress were obtained by fitting the experimental data. Based on the vertical stress–permeability functional relationship of coal masses around borehole, a numerical calculation model for methane seepage from coal masses around borehole was established, and the influences of drainage time, initial gas pressure, borehole diameter, and drainage negative pressure on the effective drainage radius of borehole were investigated. The numerical simulation results show that with the increase in initial gas pressure and borehole diameter, the effective drainage radius of borehole increases continuously but its increase amplitude decreases constantly. With the increase in drainage negative pressure, the effective drainage radius of borehole increases linearly but the increase amplitude is relatively small. The layout parameters of borehole along coal seam #9 of the Kaiyuan Coal Mine were optimized based on the numerical calculation results, and the reasonable drainage time, reasonable borehole diameter, and reasonable drainage negative pressure are 180 days, 120 mm, and 15 kPa, respectively, for the borehole along coal seam #9.