Abstract: Based on the character of the friction stir welding, the coupled thermo-mechanical visco-plastic finite element formulation was established, and the local adaptive mesh method was used for simulating the friction stir welding (FSW) processing. Based on the numerical result, the distribution of temperature, strain, stress and metal flow state of the specimen were obtained, and the defects were forecasted during the welding process. The results show that the temperature distribution of the workpiece is asymmetry; strain distribution is not uniformity along the thickness direction of the plate, and severe plastic deformation exists in the welding zone. So the FSW is 3D severe plastic deformation process, and the thermoplastic is the real principle of weld formation.