Abstract: In order to study the coal body permeability improvement effect by pressure relief ahead of mining face and improve gas drainage quantity, the paper an alyzed the correlation of coal body deformation failure and permeability change ahead of mining face. In pressure relief zone ahead of mining face, coal body slip failure occurred, which had marked expansion and permeability improvement effect. The authors tested the change laws of coal body stress ahead of working face and borehol . es gas flow with advance of the face in site, the distribution scope of the abutment pressure zone and pressure relief zone were determined. In pressure relief zone, be cause coal permeability increased, the average borehole gas flow increased by 2 ~ 3 times. Based on coal permeability increasing effect by pressure relief ahead of Co al face, pre-drainage boreholes pressure relief gas drainage quantity calculation formula with different angles were achieved on the basis of varied borehole failure dist ance and pressure relief zone width. The results showed that the larger the borehole angle was, the bigger pressure relief gas drainage quantity was. In combination wit h geological conditions of No. N2015 coal face in some mine to calculate, borehole angle could reach the max value 21.4°, compared with original coal body vertical bo reholes, single pressure relief borehole gas drainage could improve 978.5 m3, which could effectively improve the coal seam gas drainage rate.