This paper uses ALE algorithm to develop a numerical model of breaking rock under high pressure water jet, in consideration of rock in a state of high ambient pressure. The simulation results show that damage evolution speed along axial direction of rock under high ambient pressure is lower than rock without ambient pressure, and ambient pressure has less impact on the damage evolution speed along radial direction. Through analyzing damage evolution of four typical element under water jet with different speed, it shows that increasing water jet speed can improve breaking rock efficiency. The relationship between water jet speed and performance of high pressure water jet breaking rock wis explained in theory.