Abstract: The eccentric extrusion flow model of friction stir welding was proposed.The model shows that the eccentric extrusion effect of tool for softening material is the most important factor for the formation of joint.The tool with eccentric structure was designed and friction stir welding experiments were carried out,and the influence of the eccentricity of tool on material flow was analyzed.Research shows that the joint forms by the tool eccentrically extruding material in the friction stir welding process.The greater the eccentricity of the tool is,the larger the weld nugget zone of the joint is.Metal flow near the pin will form the relatively obvious layered structure in the advancing direction.The arc lines will form on the surface of the weld,which are not symmetrical,but turn to the retreating side.Angle formed in the retreating side is larger than that formed in the advancing side in the arc lines.There is a corresponding relationship between the layered structure in the advancing direction and arc lines on the surface of the weld.