The Al-4.5Zn-1.5Mg-0.5Mn aluminum alloy was welded by MIG welding in test, and the microstructure and mechanical property of joint were investigated. The results of experimentation show that the microstructure of welding metal is as-cast microstructure, and the microstructure of base metal is isometric crystal. There is a narrow fusion zone near the edge of weld seam, which consists of fine columnar grains. In the base metal and heat affected zone, the precipitation was diffuse in the intragranular, while strengthening phase η mainly focused on grain boundaries. The precipitation phases in the heat-affected zone distributed unevenly with large size, intermittent and segregation. In the welding metal, there is no precipitation phase. The microhardness on center of welding metal is lowest in joint. The tensile strength of welding joint is 272MPa, and the percentage elongation after fracture is 6.6%. The zone of fracture is located in the welding metal.