Abstract: Numerical simulation was applied to analyze the deformation of sheet aluminum alloy by friction stir welding, and the calculation result was compared with that of the real joint. The result shows that such deformation is asymmetric. The large deformation region appears on the advancing side, which results from application of the mechanical force on the joint from the tool. The deformation characteristic of the points is located in both sides of the weld showed that acute fluctuation appears on the retreating side, indicating that the stability of the retreating side is lower than that of the advancing side. The stress distribution shows that the high stress region is on the end of the weld, which attributes to the x-y shear stress.