Simulation of the dynamic characteristics of micro abrasive water jet (MAWJ) is conducted using computation fluid dynamics (CFD) software Fluent 6.3 flow solver. The velocity distributions and particle behaviors of the free jet and impinging jet in and out of the nozzle are investigated under different input and boundary conditions. In the free jet simulation, a reduction in water pressure corresponds to more rapid decay of the jet velocity along the jet axis, whereas particle mass concentration has no influence on the jet velocity. In the impinging jet simulation, the effect of the impingement surface on the flow field increases with a decrease of the stand-off distance. The simulation results in this study provide the foundation for optimizing the nozzle structure and improving cutting efficiency and cutting performance of MAWJ.