Abstract: Due to the geological condition complicated, coal soft, low seam permeability, which caused pre-gas drainage borehole jammed and collapsed and other problems, a study was conducted on the permeability improved principle and technology of the large diameter borehole. Also a site comparison test was conducted in the air returned gateway of South No. 4 Block in Shihao Mine. The results showed that comparison with the conventional borehole and single large borehole, the gas drainage volume fraction of the large diameter borehole combined with the casing protection borehole technology could be improved by 20% and 47%, the attenuation was slow and the gas drainage concentration was slowly attenuated. Within 90 days, the accumulated gas drainage volume was increased by 1.45 and 3.8 times. The average gas drainage pure volume of the large diameter borehole with the casing protection technology was 0.18 m3/min and was increased by 1.42 and 3.87 times individually. The site test showed that the large diameter borehole combined with the casing protection technology could effectively expand the pressure released area of the seam, could improve the gas seepage in the seam, could obviously improve the seam permeability, could reduce the pre-drainage borehole jammed and collapsed and could improve the gas drainage concentration and gas drainage effect.