In this paper, 6.5 mm thick Al/Cu composite plates composed of the Cu cladding plate with 30% were selectedfor friction stir welding（FSW）.The influence of welding position（Cu cladding plate was placed upon and Al base plate was placed upon the Cu cladding plate）on the welding process was investigated emphatically. In addition, the influence of thermo-mechanical coupling on the material flow of Cu clad plate, the microstructure evolution mechanisms in the welded joint, especially the interfacial microstructure were analyzed combined with numerical simulation technology. The results show that when Al base plate was placed upon the Cu cladding plate, the tunnel defect was created under the process parameters that were the plunge depth of 0.2 mm, the rotation speed of 900 r/min and the welding speed of 150 mm / min in the weld joint, because of the weakly influence of the action of thermal mechanical coupling on the Cu clad plate, the weakly interaction of upward migration, the lower temperature effect on high-melting point high-conductivity copper clad plate. When Cu cladding plate was placed upon the Al base plate, the joint with perfect appearance could be prepared under the same welding parameters. Due to the temperature variations of the advancing side and the treating side during the FSW process, the small number of Al-Cu intermetallic compound interlayer were formed in the Cu/Al interface of the lower temperature treating side and hook zone the thick Al-Cu intermetallic compound interlayer were formed in the Cu/Al interfacein the advancing side. The average tensile strength of the joint is 85.2 MPa, which is about 62.7 % of the tensile strength of the base metal.