Abstract: To address the problems of existing hydraulic intensifiers used in underground coal mines—such as large size, inability to achieve pressure boosting while drilling, and the need for auxiliary driving devices—a passive hydraulic intensifier for while-drilling operations was developed based on the piston differential pressurization principle, employing a multi-stage multi-chamber modular design and a coordinated control technique using directional valves. This intensifier did not rely on an external high-pressure pump and converted low-pressure underground mine water into high-pressure water autonomously, enabling high-pressure permeability-enhancement operations. It achieved a pressurization ratio of 1∶10 with a minimum pressurization pressure of 20 MPa. The influences of high-pressure outlet diameter, effective piston acting area ratio, and input pressure on the performance of the intensifier were analyzed: the larger the high-pressure outlet diameter, effective piston acting area ratio, and input pressure, the faster the flow velocity of the high-pressure liquid, but the longer the time required for the high-pressure liquid to reach a steady-state velocity. Field industrial test results showed that, during hydraulic punching and cavity creation in a soft coal seam with a firmness coefficient of 0.15, the average coal output per meter of borehole reached 1.5 t/m, and the intensifier exhibited stable performance, achieving pump-free high-pressure hydraulic permeability enhancement.