Abstract: In order to research the mechanism influenced by the tilt angle of tool pin during friction stir welding, the thermal-mechanical coupled model considering the tilt angle was established based on DEFORM-3D software. The three-dimensional material flow in the welding seam region was simulated, and the difference of the material flow behavior influenced by tool tilt angle of 0° and 2° was compared and analyzed. The results showed that most of the materials in the front advancing side deposited in the region of rear advancing side after rotating around the tool pin, while most of the materials in the front retreating side were pushed by the tool pin and deposited in the rear region. It was observed that the tilt of tool pin increased the fluidity of the material between the rear retreating side and the rear advancing side, meanwhile, the flowing behavior in the thickness was increased accordingly. The simulation results of the joint morphology agreed well with the corresponding experimental results, which illustrated that the numerical model of the material flow considering the tool tilt angle can be used for predicting the tendency of welding joint defect.