Abstract: Modern analytical methods have been used to investigate the patterns of fusion zone and the diffusion behaviors of elements by TIG direct welding and TIG with Sn as transition layer welding. The result shows that there is an obvious interface between Mg and Al base metal when they are welded directly. The element of Mg diffuses into Al base metal, and forms a diffusion layer, but at the same time there is only a little element of Al diffusing into Mg base metal. The Mg diffused into Al base metal can combine with Al and form Mg₁₇Al₁₂ and β-AlMg phases, which will cause the rupture of diffusion layer easily. TIG with Sn as transition layer welding, as a fusion-braze welding method, forms a Sn transition zone with different components, diffusion organizations and different performances between Sn and base metal. The Sn transition zone is narrower than the diffusion layer of TIG direct welding, which has brought constraining force on welded joint and hampered the diffusion of Mg element into Al base metal. The Sn transition zone takes the blocking effect on the direct contact between Mg and Al.