The excavation performance of Water Jet (WJ) technology is affected by many parameters. Therefore, many experiments are required for the development and application of WJ technology. If a realistic numerical simulation of WJ excavation could be produced, we might be able to identify the optimum conditions for excavation, which could reduce the cost of developing new WJ technology. In this study, we focused on the Smoothed Particle Hydrodynamics (SPH) method, which is a type of particle method, for the numerical simulation of WJ excavation of soft rock saturated with water. The main results can be summarized as follows: (1) We developed a three-dimensional SPH simulation code for WJ excavation of soft rock saturated with water. (2) We clarified the usefulness of this numerical simulation code by comparing the calculated results with those from excavation experiments in soft rock saturated with water.