Abstract: Compared with friction stir welding (FSW), the material flow behavior during the bobbin-tool FSW (BT-FSW) process is more intense and complicated due to the presence of lower shoulder, which severely affects the mechanical properties of the joint. In this paper, a 3D thermo-mechanical coupling model of BT-FSW 2219 aluminum alloy was established based on the coupled Eulerian-Lagrangian (CEL) method and the material flow behavior during the BT-FSW process was investigated by using tracer particles. The results show that the material flow in the BT-FSW process is inconsistent. The material near both shoulders moves in advance, showing violent behavior. The farther away from the shoulder, the more lagging and stable material flow. As the inner material of the shear layer, the material on the advancing side of the weld rotates around the pin in the horizontal direction and most is deposited on the advancing side area behind the tool. While the material on the retreating side is only driven by the material in the inner layer of the shear layer, and most is pushed to deposit behind the weld. In the thickness direction, the plastic material shows weak flow before reaching the welding center line behind the tool. And the material is squeezed by both shoulders to flow toward the center of the plate after it enters the advancing side area behind the tool.