Abstract: Based on thermal simulation an Arrhenius type constitutive relationship of 2024-T3 aluminum alloy, the 3D thermodynamic friction stir welding (FSW) was modeled by Deform-3D software. The physical fields including temperature, stress, strain were investigated during friction stir welding. The results reveal that all the physical fields are unsymmetrical in cross-section due to rotation speed and travel speed. The material appears compressive stress in advancing side (AS) and tensile stress in retreating side (RS), respectively. Furthermore, material respectively flows to the surface and bottom of weld in advancing side, which results in defect occurrence when the material flowing speed from RS to AS is lower than that of in thickness. The phenomenon is in good agreement with the corresponding experimental results.