Abstract: In order to determine dilling sealing length of gas drainage with horizontal drill bore, based on the stress distribution around coal roadway and the relati onship between permeability of coal seam and strain on coal-body after tunnel excavation in coal seam, theoretical analysis on the law of gas flow through coal-body W hich change with permeability of coal seam,“Constant pressure flow measurement"had been put forward to determine relaxation zone scope of coal roadway. The feasi bility of the method had been analyzed in theory and the equipment for the determination had been designed according to the principle of the method. A field experimen t was conducted in Xuehu Coal Mine, according to the measured results, steady gas injection flow suffers from attenuation gradually along with the rising on sealing de pth; the gas flow appears to be stabilizing while the bore sealing reaching a certain depth. Although the attenuation, but at a modest rate within a range of 21% to 26%, the gas flow rate was much lager in the relaxation zone. However, the fractures reduced sharply along with the rising on drilling depth and steady flow reduced at a larg e rate within a range of 71% to 82% in the transition to relaxation zone. These two regions was not suitable for sealing drainage due to much more fractures. However t he development of fractures was stabilizing in the scope of 10. 5 meters, drainage was workable. Drainage effect was investigated at site based on different bore sealin g depth. It turned out that the drainage concentration and bore life increased while bore sealing conducted on the basis of measuring relaxation zone. The problem of ai r leakage due to the unscientific bore sealing depth had been solved.