A method for enhanced gas extraction by coal mine underground gas-liquid dual-phase alternating phase-driven fracturing of a coal body. Constructed in a present coal seam or a permeable stratum are a fracturing borehole and a water guiding hole, where both boreholes are sealed to withstand high pressures. Fracturing equipment is connected, pressurized water of no more than 3 MPa is injected into the fracturing borehole, and, a water pump is shut off after 10 minutes of hydraulic fracturing of the fracturing borehole, thus stopping the hydraulic fracturing. An air pressure booster is turned on for gas-phase fracturing of the fracturing borehole, and, when the pressure reaches 3 MPa, the air pressure booster is turned off, thus stopping the gas-phase fracturing. This is repeated for multiple times, when water flows out of the water guiding hole distanced from a side of the fracturing borehole, the hydraulic fracturing is stopped, while the gas-phase fracturing is continued, and, when water ceases to flow out of the water guiding hole or when a gas erupts therefrom, the gas-phase fracturing is stopped. A fuel gas extraction pipe network is connected to the fracturing borehole and the water guiding hole for fuel gas extraction. The method effectively solves the problem of fuel gas release and analysis being impeded by residual water when only hydraulic fracturing is used, thus allowing cracks within a coal body to be fully developed to an improved degree, and enhancing coal seam air permeability and fuel gas extraction effects.
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