Abstract: The butt welding of AZ31 magnesium alloy and 6061 aluminum alloy was carried out by oscillation laser with Sn foil assistance, and the microstructure, properties, temperature and flow field distribution of Mg/Al joints and the influence of adding Sn foil on the interfacial element diffusion behavior were investigated. The results show that under the condition of "8"-shaped oscillation trajectory, when the oscillation frequency is 30 Hz, the maximum tensile load of joint is 887.14 N, when the oscillation frequency is 50 Hz, the tensile load of the joint reaches the maximum value of 936.05 N, but when the oscillation frequency continues to increase, the mechanical properties of the joint decrease significantly. The weld is mainly composed of β-Al, Sn solid solution and Mg₂Sn, Mg₁₇Al₁₂ compounds, and with the increase of oscillation frequency, the diffusion degree of Sn element increases while the diffusion degree of Mg element decreases. When the oscillation frequency is low and moderate, Sn foil plays a role in blocking, and the Sn element gathers at the Mg side, thus Mg₂Sn compound with low brittleness is formed. In addition, the weld width is uniform and the defects are few, which is beneficial to improve the performance of the joint. However, at higher oscillation frequency, the vortex effect of molten pool is enhanced, and the flow direction of molten metal becomes complex and changeable, the element distribution is disordered, thus the barrier effect of Sn foil is weakened. Moreover, the molten pool is over disturbed, the surface defects of the weld increases, which leads to a significant decrease in the performance of the joint.