Abstract: Based on the section analysis of three-dimensional model of friction stir welding, the evolution of the joint section of the welding process was established.The forming process of friction stir welding was divided into extrusion stage, migration stage, backfilling stage and shoulder effect stage.The first three stages which were affected by stir pin were analyzed.The analysis of the material migration processes of the first three stages pointed out that the original surface and the residual oxides were migrated to the retreat side in the extrusion stage, the onion ring structure was formed, the oxides were fragmentated and dispersed in the migration stage and the onion ring structure was fully formed in the backfilling stage.The studies showed that if the above three stages could not effectively complete the forming processes, the superposition effect of a variety of defects forming mechanisms would lead to the formation of the final defects.The results show that the four stages of the forming process established by using the section analysis can be used to explain the forming mechanisms of FSW joints and the causes of the "S line" and other defects.