Based on twophase flow theory and used the FLUENT fluid analysis software to do numerical simulation of inside and outside flow field in high pressure abrasive water jet, by changed the viscosity，the flow and the density of fluid, the influence in distribution of cutting force and velocity by each parameters was discussed.The results show that， density of fluid increased has improved the cutting force and jet velocity significantly; but viscosity increased has a counter effect, due to the flow resistance increased, which makes the outlet jet velocity fast decay, and reduces cutting force at outlet interface. The flow change of the working medium can also have a greater impact on the inlet velocity and jet results. An appropriate increase within the allowable range of inlet flow in the device can improve jet results, under working environment. 