Abstract: The pressure relief drilling mechanics model established by polar and rectangular coordinate system was used to analyze the relationship between the lateral abutment pressure of roadway and the stress distribution of coal body around pressure relief borehole,the unloading stress and the pressure relief coefficient of coal body were obtained.According to the stress state of the coal body before and after pressure relief,the stress field of pressure relief borehole was divided into four stress zones.They were plastic - elastic zone,elastic - plastic zone,plastic - elastic zone and elastic - elastic zone.The pressure relief was mainly in plastic - elastic zone and elastic - elastic zone.The optimal length of pressure relief borehole was analyzed combined with the stress field zoning of coal body before pressure relief.The results showed that borehole length should be designed according to the lateral abutment pressure distribution of roadway before pressure relief.The unloading stress was middle high and both sides low after pressure relief borehole,while the coal body stress remained unchanged in the area without pressure relief borehole.The numerical simulation method was used to study the pressure relief effect of different borehole length in Xuehu coal mine,then came to the conclusion that the optimal borehole length was about 20 m.