Abstract: In order to improve the efficiency of gas extraction in Zhangji Coal Mine, in view of the characteristics of high in-situ stress and low permeability of the coal seam, similar tests and engineering applications were used to study the characteristics of fracture development, expansion and evolution in the process of hydraulic fracturing coal seam, and the mechanism of pressure relief and permeability enhancement in overburden strata. The fracturing technology combining "repetitive fracturing" with "first fracturing and then flushing" through drilling holes is put forward to increase coal seam permeability. According to the simulation results, the field engineering test is carried out by selecting reasonable fracturing parameters. The results show that the hydraulic fracturing technology can effectively improve the gas extraction quantity and shorten the time of meeting the standard.