Abstract: Aiming at the problems of poor permeability, high gas content and inefficient gas drainage from in-seam boreholes in the No.3 coal seam of Zhaozhuang Coal Mine, ultra-high pressure hydraulic slotting combined with cross-measure boreholes was proposed to enhance the permeability of the coal seam. Through field tests, the reasonable technological parameters of ultra-high pressure hydraulic slotting for the No.3 coal seam were determined, and a comparative analysis was conducted on the gas drainage volume fraction, pure gas drainage volume, time to meet gas drainage standards and residual gas content in the coal mass after drainage between the slotted zone and the unslotted zone. The results show that the optimal combination of technological parameters for ultra-high pressure hydraulic slotting in the No.3 coal seam of Zhaozhuang Coal Mine is as follows: slotting pressure of 80 MPa, slotting duration of 9 min, slotting rotational speed of 60 r/min and slot spacing of 1 m. Field application results indicate that the gas drainage volume fraction of ultra-high pressure hydraulic slotted cross-measure boreholes and ordinary cross-measure boreholes ranges from 17.21% to 51.86% and 12.48% to 34.12%, respectively, with the average single-borehole gas drainage volume fraction being 33.00% and 22.20%, respectively; the gas drainage volume fraction of ultra-high pressure hydraulic slotted cross-measure boreholes is 49% higher than that of ordinary cross-measure boreholes. The pure gas drainage volume of ultra-high pressure hydraulic slotted cross-measure boreholes and ordinary cross-measure boreholes ranges from 0.010 m³/min to 0.015 m³/min and 0.003 m³/min to 0.005 m³/min, respectively, with the average single-borehole pure gas drainage volume being 0.012 m³/min and 0.004 m³/min, respectively; the single-borehole pure gas drainage volume of ultra-high pressure hydraulic slotted cross-measure boreholes is 185% higher than that of ordinary cross-measure boreholes. The time required to meet gas drainage standards for the coal seam is 54 d and 154 d for ultra-high pressure hydraulic slotted cross-measure boreholes and ordinary cross-measure boreholes, respectively, representing a 65% reduction. After 54 days of drainage, at distances of 1.5, 2.0 and 2.5 m from the gas drainage boreholes, the residual gas content in coal around ultra-high pressure hydraulic slotted cross-measure boreholes is 6.886, 7.538 and 7.952 m³/t, respectively, while that around ordinary cross-measure boreholes is 11.442, 11.886 and 12.262 m³/t, respectively. The residual gas content in coal around boreholes decreases significantly after hydraulic slotting treatment.