The numerical model of rock breaking by water jet was established based on finite element method (FEM) and smoothed particle hydrodynamics (SPH) to research the efficiency of rock breaking by water jet. Firstly, the research on the impact pressure of water jet was conducted under different diameters and velocities. Then, the effect of water jet diameter, jet angle and jet velocity on the efficiency of rock breaking was studied and the reasons were shown through analyzing the impact rock momentum, rock energy, mean cutting depth and mean cutting width. The results indicated that, the efficiency of rock breaking increased with the jet angle firstly and then decreased, and the jet angle about 10 degrees was the most optimal to break rock; meanwhile, it can be concluded in terms of energy that more micro-damages formed when the jet angle was 0 degrees. The efficiency of rock breaking, expressed as the relative rock eroded element number, mostly increased with the diameter of water jet in a trend of stair-step and there was no difference under different rock strength. Moreover, rock breaking efficiency increased with the velocity of water jet in a trend of straight line and the trend varied not with rock strength yet. However, the trend slowed down with the increase of rock strength, which showed that efficiency of rock breaking could be enhanced greatly by increasing velocity of water jet, and the smaller the rock strength, the more obvious the efficiency. (C) 2015 Elsevier B.V. All rights reserved.