In order to improve the cutting performance of the post-mixed abrasive water jet,the mathematical model of the abrasive motion in the focusing tube was established and numerically simulated on the basis of theory of solid fluid two phase flow by using FLUENT software module. The traditional control model of particle phase was optimized by a researching object of single particle in the focusing tube,and the basic movement condition of abrasive particle in the nozzle was obtained. Research results show that the abrasive particles join the high-speed jet by the collision between each phase after they enter into the mixing chamber. Therefore two phase jet flow in the end is formed. Under the action of high speed water jet carrying,the speed of the particles will rise rapidly on initial forming stage of the two phase flow. Due to causes of sudden change of internal structure in mixing chamber,phase velocity of particle has a big difference and is unsteady. After the particles enter into the focus tube through convergence section,the phase velocity of particle will exponentially approach the phase velocity of liquid jet and it will gradually be in steady. But because of causes of linear energy loss along the path,phase velocity of particle has always slide away relatively to that of the liquid jet. The velocity of abrasive particles is slightly slower than liquid jet speed after all. 