Abstract: China has a wealth of abundant coalbed methane (CBM) resources at great depths. However, the low permeability of these deep coal seams coupled with the challenges posed by high-stress pressure environments, hampers efficient gas extraction. Water jet punching and cavitation in horizontal wells is a new technology for relieving pressure and enhancing the permeability of coal seam over a wide area. Nevertheless, the mechanisms underlying rock breaking and pore formation in deep environment remain to be studied. Because the deep coal seam exists in the geological environment of high ground stress and high stress difference, it is difficult to carry out the physical simulation experiment in the laboratory. Consequently, a numerical model of water jet punching and rock breaking is established based on the viscoelastic-plastic theory and material point method. The model integrates the advantages of the Lagrange and Euler algorithms, proficiently simulating the whole process of momentum exchange, large deformation of coal, rock breaking, fluid erosion and rock carrying and reverse drainage throughout the process of jet rock breaking. Subsequently, based on this numerical model, the mechanism and influencing factors of rock breaking and pore-forming by water jet in deep coal seams are studied, and the conclusions are as follows: ① the shear failure caused by high in-situ stress load in deep coal seams is the leading mechanism of rock breaking, while jet mainly plays the role of erosion and rock carrying and reverse drainage. The tensile failure caused by stress wave generated by water jet collision with coal rock is the leading mechanism of rock breaking when there is no stress load. Due to the strong plasticity of coal rock, water wedge effect is not obvious. ② The high in-situ stress load accelerates the rock breaking process of deep coal seams. The amount of rushed-out coal under high in-situ stress load is higher than that without stress load. Meanwhile, the stress load causes the coal body to move towards the opening direction, resulting in a decrease in cavity volume, but the plastic damage zone is significantly increased compared with that without stress load. ③ The amount of rushed-out coal and the area of plastic damage zone increase with the increase of jet time, but the increase rate gradually slows down. The amount of rushed-out coal and the area of plastic damage zone show a downward trend with the increase of incident angle. The smaller the jet angle, the better the effect of water jet punching and rock breaking. The vertical incidence of jet is not the best way to break rock. ④ The amount of rushed-out coal and the area of plastic damage area increase with the increase of stress difference and the width of coal-rock opening, but the increase of stress difference has little effect on jet rock breaking in deep coal seams. The width of coal seam opening should not be too high, and the rock breaking efficiency of jet can be improved by increasing the width of the opening hole appropriately.