Abstract: To address the difficulty in gas extraction from low-permeability coal seams, In view of the problems of poor drilling performance of water Jet and weak rock-breaking and slag discharge capabilities of abrasive water jet, The design and analysis of a water jet/ abrasive water jet composite self-propelled drilling device Were Conducted. The device is mainly composed of a self-propelled nozzle, inner and outer coaxial composite pipelines, and other components. The forward nozzle of the self-propelled nozzle employs a swirling element to generate a rotating abrasive water jet, while the backward nozzles adopt a uniformly distributed circular arrangement of multiple nozzles to form a water jet. The water-abrasive two-phase flow is transported by the inner pipeline of the inner and outer coaxial composite pipelines to the forward nozzle of the self-propelled nozzle, where an abrasive water jet is formed, thus enabling the drilling function of the device; High-pressure water is conveyed by the outer pipeline to the backward nozzles of the self-propelled nozzle, where a water jet is formed, achieving the reverse spray self-propelled and slag discharge functions of the device. Based on fluid mechanics theory, the initial self-propelled force of the water jet/ abrasive water jet composite self-propelled drilling device was calculated; the fluent software was used to obtain the flow velocity distributions for different nozzle diameters, and the erosion diameter and initial self-propelled force were analyzed. Self-propelled drilling performance tests are conducted, The tests show that:Under the conditions of a working pressure of 24 MPa, a dual-flow-groove pitch of 8 mm for the swirling element, a backward nozzle inclination angle of 20°, and four backward nozzles, an erosion diameter of 45 mm is achieved when the forward nozzle diameter is 2.5 mm and the backward nozzle diameter is 1.5 mm, meeting the requirement for the maximum outer diameter (36 mm) of the water jet/ abrasive water jet composite self-propelled drilling device.The drilling distance reaches 8.6 meters, which meets the requirement of a 5.0-meter drilling distance. In the theoretical calculation of the recoil force of swirling jets, a correction coefficient“K”is introduced to correct the recoil force based on the axial velocity of swirling jets, thereby offsetting the impact of directly using the axial velocity of ordinary jets to replace that of swirling jets. The error between the simulated initial self-propelled force (202 N) and the experimental initial self-propelled force (186 N) is approximately 8.69%, showing good consistency, which can verify the feasibility of the composite self-propelled drilling scheme with pure water jet/abrasive water jet. The composite self-propelled drilling method with pure water jet/abrasive water jet proposed can effectively improve the efficiency of self-propelled drilling and provide a new approach for coal-rock drilling technology.