Abstract: A three-dimensional mathematical model of internal flow field in a soil bore-enlarged nozzle was established with the CFD software. Using the standard κ-ε turbulent model, the internal flow field in the nozzle was simulated and the influence of nozzle parameters on the velocity contour, pressure contour and outlet axial velocity of the flow field was studied. The results show that the diffuse angle and divergent cone length of the nozzle influence the internal flow field more and the effect of the cylindrical segment length is relatively less. The optimum values of all these parameters were proved to exist. Simulated results close to indoor test results, which accounts for the relationship between the internal flow field distribution in the nozzle and the soil impact effect of water jet.