This paper studies the mechanism and process simulation of removal of molding sand by a high pressure water jet using a nonlinear dynamic damage model and finite element analysis (FEA). The results indicate that a high pressure water jet possesses a strong ability to remove molding sand. The destruction of molding sand is a concurrent result of stress wave and quasi-static pressure, in which the alterative stress wave plays a major role. The numerical results agree with the test results, indicating that the model and FEA are feasible for analyzing the mechanism and process of removal of molding sand by a high pressure water jet. These conclusions can be used for equipment design and process optimization of removal of molding sand for precision castings.