In order to assemble and connect the dissimilar aluminum alloy ring parts accurately, a horizontal friction stir welding tool which can change axial movement into radial expansion was designed. The results show that the Al-Cu/Al-Mg dissimilar aluminum alloy ring parts can be assembled and connected with high precision and high quality. The internal quality of the joint meets the requirement of grade I. The tensile strength and elongation of the joint are higher than 245 MPa and 16%, respectively. Besides, all the joints are broken on the side of Al-Mg based aluminum alloy. It can be concluded that the overall performance of the joint is better than that of Al-Mg based aluminum alloy. Finally, the engineering problem of welding cracks during the Al-Cu/Al-Mg dissimilar aluminum alloy welding is solved.