Using 8 mm length friction stir tool,7055-T6 aluminum alloy with the thickness of 12 mm was double-sided welded by friction stir welding（FSW）,and the microstructure of the welded joint was observed,meanwhile,the mechanical properties were tested.The results show that the maco morphology of weld joint has no obvious plastic flow trace. When the AS on the same side,the tensile strength of the welded joint can reach 452 MPa,the joint coefficient of strength can reach 75%,which is better than the AS on the different side. The horizontal microhardness of weld is "W" distribution. The minimum of the microhardness is 101.7 HV on transition zone between TMAZ and BMZ of RS. The vertical microhardness of weld shows that the front side is lower than the back side. The morphology of tensile fracture surface appears dimples ductile fracture characteristice,which is ductile rupture.