Abstract: Objective The aims is to study flow behavior of material in friction stir welding (FSW) process and understand mechanism of FSW process. Methods By ABAQUS software, numerical model of friction stir welding of AZ31B magnesium alloy is established based on smoothed particle hydrodynamics (SPH) method, and trend of material flow is studied by tracking characteristic particles. Results The results show that displacement of material flow in the forward direction of welding is larger than that of the other two directions, advancing side is larger than retreating side, and material flow performance of retreating side is weaker than that of advancing side. Material moves around stirring head, flowing from advancing side to retreating side. Deformation and displacement of particles closer to stirring head part are greater. Particles on the retreating side spiral up along the axis of stirring head, and finally stay on the retreating side and mix in the rear side of stirring head. Conclusion Generation of flash and cavity defects in the FSW process is predicted. Ratio of kinetic energy and internal energy verifies feasibility of the model.