Abstract: The finite element model of 5052 Al alloy friction stir welding was established by using Abaqus software based on ALE（arbitrary Lagrangian-Eulerian） method, and the material flow in the process of 5052 Al alloy friction stir welding was simulated by particle tracking method. The results show that the material in the advance side enters the retreat side with the rotation of the shoulder, but the material in the retreat side will not enter the advance side. The upper and lower surface materials will not leave their respective surface layers, while the middle layer materials enter the upper surface layer driven by the shaft shoulder. With the increase of rotation speed, the material flow ability increses. The simulation and test results show that compared with the tool with smooth shoulder, the tool with concave shoulder can effectively avoid the hole defects of the joint under the same welding parameters.