Abstract: The eccentric squeeze by shoulder plays a decisive role in arc lines formation process of aluminum alloy friction stir welding. The arc lines formsbecause of the softened material squeezed by long axis which generates by eccentric of tool in joint area. Peaks of arc lines form by squeezing of long axis of tool, and there are valleys of arc lines between two peaks of arc lines. The eccentric extrusion flow model was proposed. Formation of arc lines can be characterized by numerical model. Friction stir welding process by using tool without pin can get arc lines by numerical simulation way. Friction stir joining processes withand without pin tool were contrasted by experimental methods. Research shows that arc lines form in the process and the contour of arc lines without pin is the same as that with pin. The eccentric squeeze by shoulder plays a decisive role in arc lines forming process. Faster advance speed and slower rotation speed of tool lead to the long distance between the arc lines. Angle of arc line formed in the retreating side is larger than that in the advancing side.