Abstract: There are some bottleneck problems in the pressure relief and antireflection technology of submerged water jet in a downward hole, such as weak impact force, low coal breaking efficiency, less coal cutting and poor adaptability. In this study, the idea and principle of coal breaking and antireflection of self-excited pulsed water jet in downward hole strengthened by the annular fluid were put forward in order to break the pressure holding effect at the bottom of the hole and the drag increase effect of water cushion, and improve the impact force and coal breaking efficiency of the submerged water jet. The high-efficiency modulation and coal-breaking characteristics of self-excited pulsed water jet enhanced by annular fluid were studied by using large eddy simulation, impact force test and impact kettle coal breaking experiment. Firstly, the physical model of an annular fluid-enhanced self-excited pulse nozzle suitable for the anti-reflection of downward holes in coal mines was established, and the nozzle structure parameters were optimized. Then a high-frequency impact force test system of a submerged pulsed jet and annular fluid-enhanced self-excited pulsed water jet nozzle was developed, and the impact force and flow field characteristics of the annular fluid-enhanced self-excited pulsed nozzle in the submerged environment were quantitatively analyzed. Finally, the coal breaking experiment system of submerged water jet was developed, and the coal breaking experiment of submerged water jet was carried out, which revealed the coal breaking characteristics of the annular fluid-enhanced self-excited pulsed water jet. The results show that under the engineering condition of a down-hole in the coal mines, the self-excited pulse water jet enhanced by annulus fluid can be effectively generated by the self-excited pulse nozzle with self-priming Helmholtz resonance cavity structure on both sides. The optimal nozzle structure parameters are as follows: the diameter of the upper nozzle d₁=3 mm, the angle of collapse wall α=120°, the diameter of the lower nozzle d₂=3.6 mm, the length of the oscillation cavity L=10.5 mm, the diameter of the oscillation cavity D=27 mm, the diameter of the inlet port d₃ =4.2 mm. When the pump pressure is 10 MPa, the maximum amount of entrainment annulus water is 0.28 L/s, with an average of 0.19 L/s. When the pump pressure is the same, the dominant frequency of the annular fluid-enhanced self-excited pulsed water jet is close to that of the self-excited pulsed water jet. The dominant frequency range of the annular fluid-enhanced self-excited pulsed water jet is 253−285 Hz, while that of the self-excited pulsed water jet is 251−282 Hz. The peak impact force of the annular fluid-enhanced self-excited pulsed water jet is 1.05−1.08 times and 1.63−1.89 times higher than that of the self-excited pulsed water jet and the continuous jet, respectively. The average impact force is 1.09−1.16 times higher than that of the self-excited pulsed water jet, which reflects that the annular fluid-enhanced self-excited pulsed water jet can continuously generate a large impact force in the submerged state and has good pulsed jet characteristics. Under the condition of the same pump pressure and target distance, the damaging effect of annular fluid enhanced self-excited pulsed water jet is stronger than that of self-excited pulsed water jet and continuous water jet on coal samples. The annular fluid enhanced self-excited pulsed water jet has a good erosion effect on coal body, and about 88.65% of the coal slag particle size is below 8 mm. When the pump pressure is 10 MPa, the target distance is 10d₁, and the erosion time is 10 s, the jet has the best erosion effect on coal samples, with an erosion depth of 47.32 mm and the erosion volume of 15.7 cm³. The research results can not only solve the bottleneck problem of a downward hole water jet in the coal mines, but also have the advantages of less excavation and cost saving. It is expected to replace and exceed the pressure relief and permeability enhancement of the upward hole water jet. It has broad application prospects in the gas disaster prevention and coalbed methane development of coal mines.