Aluminum alloy has being widely used in modern automobile and aeronautic industry. However, the welding of aluminum alloy, especially high strength aluminum alloy，is difficult. Porosities are usually brought in the weld metal. In this paper, MIG welding using mixed gas shielding is carried out. The characteristic shapes of porosity in weld metal are described, the mechanism of porosity formation is analyzed, and the factors that influence the tendency of porosity formation are studied. Experiment results indicate that by the use of mixed shielding gas of 38%He+62%Ar, the number of porosity is reduced, the width of HAZ and softened zone is decreased, and the mechanical properties of welded joint is increased.