To apply the high pressure water jet technology to the engineering practice of waste ammunition treatment project, the process of breaking explosives by high pressure water jet was studied by the numerical simulation method. A finite element calculation model with high pressure water jet velocity as the input parameter was set up. The Lagrange algorithm and SPH（Smoothed Particle Hydrodynamics）algorithm were used to solve the problem by DYNA solver. The safety for the process of breaking explosives by high pressure water jet was studied through the analysis of pressure, temperature and reaction degree of explosive, and the effectiveness for the process of breaking explosives by high pressure water jet was analyzed through the number analysis of failure unit. Results show that when the velocity of high pressure water jet is lower than 800 m·s^-1, the high pressure water jet can be used to break the charge of waste ammunition explosive, and ensure the safety of the breaking process. When the velocity of high pressure water jet is 150~350 m·s^-1, the large cross-section breaking can be performed more effectively; when the velocity of high pressure water jet is 400~800 m·s^-1, the small cross-section breaking can be realized efficiently.