Abstract: The finite volume model of friction stir welding was established on basis of the Fluent software,which considered the practical structure of rotational tool and the relation between the material parameters and temperature. The effect of pin structure on material plastic flow behavior was studied by numerical simulation. The results show that the flow velocity of material decreases with the increase of the distance away from the weldment surface and rotational axis of pin. The flow velocity of material inside weldment increased with decreasing of the cone angle of pine or decreasing of the width of screw pitch. As to the pin with left screw whose rotational direction is clockwise,the flow direction of material near pin is downward while the flow direction near the thermal-mechanical affected zone is upward,which is opposite to that of the pin with right screw.