1.Chongqing Energy Investment Group Technology Co.， Ltd.，Chongqing ，China；2.China Coal Technology Engineering Group Chongqing Research Institute，Chongqing ，China；3.State Key Laboratory of Gas Disaster Detecting，Preventing and Emergency Controlling，Chongqing ，China

The horizontal sublevel mining of steeply inclined extra thick coal seams has large extraction-caving ratio and complex gas emission.The proportion of gas emission from the lower part of pressure relief coal in total face gas emission is extremely high， which could result in exceeding gas limit in the upper corner of working face during mining.This paper is devoted to solve this issue using No.5754504 working face of Wudong Coal Mine of Shenhua Xinjiang Energy Co.， Ltd.as a case study.The paper analyzed the failure law for the floor and lower coal seam theoretically， computed the failure depth，optimized the layout of drainage boreholes of the lower part of working face，and analyzed the characteristic variations of pressure relief interception gas drainage boreholes with face advance.The results show that the mining damage depth of No.5754502 working face is 11.88 m，the vertical failure depth of the lower coal seam is 7.38 m，and the greatest damage depth is 10.3 m from the end of the working face.With face advance，the boreholes in the lower coal seam enter the pressure relief gradually and the gas concentration and gas flow rate of pressure relief drainage are much higher than those of pre-mining.Compared to pre-pressure relief， the effect of pressure relief is significant which is evidenced by the increase of gas flow rate of 3.2 times on average.The gas extraction ratio of No.5754502 working face exceeds 59.6% and the gas concentration in the upper corner is lower than 0.8%.The results indicate that this method can control the gas emission under limit.The safe production of coal has been realized successfully.