The friction stir welding was carried out on T-joints of 4 mm+8 mm plate-thickness dissimilar aluminum alloy 5083-O/6082-T6 by using different rotation speed and welding speed,and the macrostructure and mechanical properties of the welded T-joints were compared and analyzed. The experimental results show the T-joints of dissimilar aluminum alloy with friction stir welding are completed,and formation of weld is well and the effective connection between the skin plate and the rib plate is realized. The advancing side area of weld is in a "sickl-shaped",and that lap interface of the skin plate and the rib plate in the retreating side area is offset toward the skin plate in a "wave shaped". The weld surface morphology and bending properties of the joints are well,and the joint tensile strength of T direction reached up to 256 MPa and the fracture occurs in heat affected zone of retreating side,and the joint tensile strength of L direction reached up to 105 MPa and the fracture occurs at the junction of the lap,at the rotational speed of 1 200 r/min for welding speed of 500 mm/min. T-joint skin plate area hardness value between 70～85 HV,and the hardness value of the rib plate area decreases first and then increases with the distance from the center of the skin plate,and the minimum value is 70 HV.