The shot velocity is the key factor to determine the performance of pre-mixed waterjet. The three-phase simulation model of a mixed jet was constructed by the CFD-DEM bidirectional coupling method. The VOF model was used for a gas-liquid phase and the volume fraction influence of a discrete phase was considered. The shot solid phase was modeled by the discrete element method under Lagrangian coordinates. The two-way coupling was realized through a special coupling interface, and the simulation value of shot velocity in a fixed position range of the exit was obtained. By using the particle image velocity (PIV) test system, the shot velocity at the corresponding position was measured. By comparison, the difference between simulation and testing results was less than 6%, implying correctness of the simulation model. The simulation results show that the larger the nozzle inlet pressure and nozzle outlet diameter, the greater the shot velocity; The larger the mixing ratio (volume fraction of shots), nozzle aspect ratio and nozzle cone angle, the smaller the shot velocity. This provides a theoretical basis for the subsequent formulation of a modification process.