Abstract: A force analysis was made to the impurity particles in Al melt in steady magnetic field to set up the movement models of these particles in different flow field, thus determining their final velocities under the action of magnetic force by theoretic calculation. According to the different flow states of aluminum melt, the models of piston flow and trajectory were used to obtain the separating efficiency of impurity particles under the action of magnetic field, respectively. The results show that the motional velocity and separating efficiency of inclusion particles relate closely to their volume, flow state, magnetic susceptibility of inclusions, magnetic field intensity and the width of separation passage. The larger impurity particles have higher separating efficiency with the same motional velocity of Al melt, and reducing the motional velocity of Al melt benefits the separating efficiency.