Abstract: Technology of slotting and improving coal seam permeability with high pressure water jet is an effective method to improve gas exhausting rate, where the nozzle is the actuator of the cutting. The structural factors affecting the nozzle cutting performance are the length of stability section, the contraction angle and the length of the straight section, the three factors are mutually constrained, and an optimal value should exist. The orthogonal design method was used to simulate the flow field at nozzle outlet under different parameter combinations in this paper, and the simulation results show that the slotting nozzle with the stability length of 0 mm, contraction angle of 30 ° and straight section length of 9 mm has the best injection performance. The cutting performance was tested in laboratory, and the results show that the cutting ability of optimized nozzle was 1.5 times that of the original nozzle.