Microstructure and mechanic properties of Al-Mg alloy sheet joint were investigated by tensile, bending and hardness tests as well as metallographic and transmission electron microscope analysis. The results show that the joint of Al-Mg alloy sheet welded with Al-Mg-Zr wire by MIG has good welding quality, and the welding joint has good mechanical properties with the welding strength coefficient of 85%. The hardness value of welded joint is within 84~104, of which the lowest is in weld center. Minor Zr can improve significantly the mechanical property and resistance thermal crack of welded joint, because of the fine grain strengthening and precipitation strengthening by Al3Zr.