In order to solve the problem of the dispersion of the jet and the random distribution of the abrasive in the processing of abrasive water jet, a method of adding auxiliary magnetic field at the focus tube is adopted. Numerical simulate of the flow field distribution of magnetic field assisted abrasive water jet was carried out by help of COMSOL Multiphysics software. The turbulence model, particle tracing modules and magnetic field modules were used to analyze the movement of magnetic abrasive under the effect of magnetic field, gravity field and flow field. The simulation results show that the jet divergence become weaken and the collimation is enhanced when the auxiliary magnetic field is applied. The micro magnet abrasive is gathered along the axis of the jet by the coupling of magnetic field, gravity field and flow field, and the agglomeration effect of the abrasive is achieved. The simulation results show that the auxiliary magnetic field helps to improve the processing efficiency of abrasive water jet and the processing surface quality.