Abstract: Based on the abstract and assumption of friction stir welding (FSW), the fundamental model of plastic material flow in FSW process was built. A series of new conceptsare defined, such as step plastic deformationlayer, step cavity, stripping side and stacking side. The step plastic deformation layer is supposed as the basic unit of plastic deformation material flow,step plastic deformation layer continuously transfers into and fills the step cavity, a cycle of plastic deformation material flow stops and the next cycle starts. Driving force and resistance force related to step plastic deformation layer were analyzed, which points out that volume defects appear if the step plastic deformation layer cannot fill all step cavity. During the step plastic deformation layer transferring into the step cavity,the phenomenon of turning round to change direction will occur.