Abstract: The severe decay of gas concentration and the low efficiency of the extraction system are urgent technical problems in China’s coal mine gas extraction. A mathematical model for coal breaking by water jet was established based on the jet structure of water jet in the air. The criterion for coal fragmentation strength under water jet impact was proposed using cavity expansion theory, and the calculation method for coal breaking depth by water jet was obtained. Then, the mechanism of water jet drilling for repairing and increasing permeability in gas extraction boreholes was explained based on the flushing and unblocking effect, hole enlargement and permeability enhancement effect, and impact damage effect of water jet on coal around the boreholes, and a calculation model for the anti permeability range of coal around the borehole was established, and the distribution law of radial stress and tangential stress of coal around the borehole under the action of water jet was obtained. The finite element simulation software LS-DYNA was used to numerically simulate the coal breaking process of water jet, and the variation law of coal breaking depth under the action of jet pressure, as well as the distribution law of coal radial stress and coal damage, were obtained. Finally, the repair and penetration enhancement device for the gas extraction borehole was developed and practically applied in the penetration borehole of 14250 lower bottom lane of Xin’an coal mine. The results show that: the depth of coal breaking by water jet increases linearly with the increase of jet pressure, decreases linearly with the increase of jet distance, and decreases in a power function form with the increase of coal strength. The anti permeability effect of water jet on gas extraction drilling includes three aspects: flushing and unblocking effect, hole expansion and pressure relief effect, and impact damage effect. The water jet effect expands the drilling radius, and the anti permeability range increases linearly with the increase of drilling radius, and the higher the water jet pressure, the greater the radial and tangential stresses in the coal body, and the larger the range of coal damage around the borehole. A gas extraction drilling water jet hole expansion repair and permeability enhancement effect evaluation system has been formed based on indicators of the depth of the nozzle entering the coal seam section, the amount of coal and rock washed out, and the gas concentration. According to the results of the repair and anti permeability experiment, the ratio of the nozzle entering the coal seam depth reached 90.5%, and the amount of coal and rock ejected from a single hole could reach 385.6 kg. The implementation of the drilling expansion repair and permeability enhancement technology increases the equivalent radius of the borehole from 56.50 mm to 114.02 mm, the radius of pressure relief from 67.80 mm to 136.82 mm before the repair, and the gas volume concentration from 1.40% to 13.46% before the repair, which achieves the flushing and unblocking effect of gas extraction drilling, the enlargement of the surrounding permeability range through hole expansion, and a significant improvement in gas extraction efficiency. The research results can provide a theoretical basis and technical support for improving the utilization efficiency of coal seam gas extraction boreholes.