Abstract: By adjusting shoulder plug depth,its effects on joining performance of 3mm thick AA2024/AA2024 friction stir welded joint were studied.The results show that tunnel-type defects exists at the stirring pin advancing side in joint when the shoulder is not pressed.As the shoulder plug depth increases, the tunnel-type defects disappear and oxide defects (commonly known as S line) begin to appear.The number of oxide defects increase firstly and then decreases.S line also appears at the bottom of the weld nugget zone,and becomes gradually longer with shoulder plug depth increasing.The tensile test indicates that the highest tensile strength of AA2024/AA2024 FSW joints is 370 MPa at the welding speed of 100 mm/min,pin rotational speed of 800 r/min and the shoulder plug depth of 0.2 mm,and the highest reduction of area is 3.4%.