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Nowadays, the water jet technique has been more frequently used for coal bed methane stimulation. As the ultimate component of the water jet system, the nozzle has important influence on the rock breaking ability of water jet. To optimize the nozzle, a simplified model for impinging water jet was established. The effects of geometric structure (cylindrical, conical, and conical‐straight) and parameters (outlet diameter d, conical angle α, and length L) on the flow field of the impinging water jet are simulated through computational fluid dynamics software Fluent14.0. Besides, the effects of inlet pressure (Pin) and target distances (S) on the jet dynamics are also discussed based on the simulation results. The results show that the jet dynamic parameters including dynamic pressure, velocity, and impinging pressure vary with different nozzles. The jet dynamic parameters linearly increase with the inlet pressure, while they first increase then decrease with the increase in target distance. Additionally, the optimal nozzle designed based on the numerical simulation was used in the field test, and the results indicate that this technique can effectively improve the gas drainage efficiency. [ABSTRACT FROM AUTHOR]