To reveal the effects of the hydraulic parameters and rock properties on the characters of the high-pressure cavitating water jets and to research the mechanism of the rock-breaking ability of the water jets affected by these characters, experiments of figureing out the rock breaking effect of the water jet on the rocks with different porosity rate,were carried out under the different confining pressures and pump pressures. The results showed that the rock breaking efficiency (erosion quality and erosion depth) increased along a conic with the pump pressure increased while decreased along another conic with the confining pressure increased. The length of the bubble cloud, which decided the valid stand-off distance and affected the rock breaking efficiency, decreased along a cubic with the confining pressure increased and increased with the pump pressure increased. Under the experimental condition, when the pump pressure was 7 MPa, the cavitating phenomenon appeared and the sandstone which uniaxial compressive strength was 96 MPa could be broken. It was concluded that the rock breaking ability of the water jets was affected by the porosity rate of the rock, and higher the porosity of rock is, the more easily the rock could be broken.
