Abstract: Aiming at the problems of large volume, numerous vulnerable parts and high maintenance cost of high-pressure pumps for hydraulic fracturing in underground coal mines, a passive hydraulic self-pressurization technology was proposed, and a small-diameter modular bottom-hole self-pressurization drilling tool was developed. Through multi-stage multi-chamber modular design and directional valve coordinated control technology, the low-pressure water in underground coal mines can be autonomously converted into high-pressure water without relying on external high-pressure pumps, achieving high-pressure hydraulic fracturing operations. The influences of high-pressure outlet diameter, piston area ratio and input pressure on the working performance of the pressurizer were analyzed, and field tests were conducted. The results show that the larger the high-pressure outlet diameter, the piston effective action area ratio and the input pressure, the faster the flow rate of the high-pressure liquid after pressurization, but the longer the time for the high-pressure liquid to reach a steady-state flow rate. In the hole drilling and cavity creation test in soft coal seams with a firmness coefficient of 0.15, the average coal output per meter of a single hole reached 1.5 t, and the hole diameter was expanded by 5.5 times, achieving pump-free high-pressure hydraulic fracturing operations.