Abstract: Aiming at the technical problem of low permeability during coal seam gas extraction in Hujiahe Coal Mine, an permeability enhancement method coupling hydraulic fracturing and ultrasonic excitation is proposed. Through the synergistic effect of macro-fracture propagation by hydraulic fracturing and micro-pore modification by ultrasonic excitation, this method aims to achieve effective modification of the full-scale pore-fracture structure of coal, providing a new technical approach for efficient gas extraction from low-permeability coal seams. Taking coal samples from the No.4 coal seam of Hujiahe Coal Mine as research objects, the evolution characteristics of coal pore structures under three conditions (raw coal, ultrasonic excitation under normal water pressure, and ultrasonic excitation under water pressure) were systematically compared and analyzed based on mercury intrusion porosimetry, low-temperature liquid nitrogen adsorption experiment, and fractal theory. By quantifying key parameters such as pore volume, specific surface area, and fractal dimension in different pore size ranges, the micro-permeability enhancement mechanism under the coupled action of water pressure and sound field was revealed. The results show that the raw coal is dominated by closed and semi-closed pores with poor connectivity, and more than 90% of the pore volume and specific surface area are distributed in micropores and transition pores. After ultrasonic excitation, the pore structure is significantly improved. Under ultrasonic excitation in a normal water pressure environment, the mercury intrusion volume and nitrogen adsorption capacity increase by 34.40% and 42.14%, respectively, and the pore volume, specific surface area, and fractal dimensions D₁ and D₂ of micropores transition pores increase by 44.03%, 39.72%, 0.85%, and 0.83%, respectively. Under ultrasonic excitation in a pressurized water environment, the nitrogen adsorption capacity increases by 48.00%, and the pore volume, specific surface area, and fractal dimensions D₁ and D₂ of and transition pores increase by 47.50%, 51.79%, 1.32%, and 1.03%, respectively.