Abstract: Based on the section analysis of three-dimensional model of friction stir welding, the moving expressions about feature points on section contour of probe were established.According to the basis of mathematical analysis, the forming mechanisms of hook, cold overlap-defect and onion structure were revealed. In the light of the above research results, the optimization idea of probe shape was proposed. Considering improving plastic metal migration behavior, the s-line of large thickness aluminum alloy friction stir welded butt joint and cold overlap-defect in lap joint of stringer and plate was effectively eliminated by three aspects design. The results provide technical support for the development of friction stir welded products.