Abstract: AA7075-T6 thick plates with 20 mm thickness were friction stir welded by using tools with various pin surface profiles, and the effect of pin surface profile on flow behavior of plastic material along axial direction of weld was analyzed. The results showed that, when the tool with a taper thread pin was applied, the area of nugget zone and the transfer height on two sides in the welds increased firstly and then decreased with increasing rotation speed, while the area of loose zone firstly decreased and then increased. Moreover, the transfer height of plastic material on the advancing side was bigger than that on retreating side in the same weld. As a rotation speed of 375 r/min was used, the biggest area of nugget zone and transfer height on two sides while the smallest one of loose zone were obtained. However, the transient cavities can promote the sucking-extruding effect along the horizontal direction of the weld when the tool with a triangle taper thread pin was used during FSW. Also, the flowability of plastic material was improved along axial direction. Therefore, the loose defect in the weld was removed.