Abstract: Diffusion couples of Al-Li/Al-Li and Al/ Al-Li were prepared by using gallium as interlayer under mechanical compression force. Later heat treatment in vacuum was carried out in a superconducting magnet and the joining property was studied by shearing test. Interfacial microstructure and element content distribution of Mg near the interface were studied by EBSD and EDS, and the diffusion coefficient of Mg element in 1420 Al-Li alloy to 1060 pure aluminum was calculated. The results show that a great improvement of the joining strength was found after the heat treatment under high uniform magnetic field. The shearing strength of the joint reaches to the highest of 188MPa, which is 94.2% of the base metal and 168% improved than the joints heat treated without high magnetic field at the heat treatment parameters of temperature 520℃, magnetic flux density 12T and time 1 h. What's more, the diffusion distance of Mg in 1420 Al-Li alloy to 1060 pure aluminum becomes longer and diffusion coefficient becomes larger with the increase of the magnetic flux density. The diffusion coefficient is improved by 25% under the magnetic flux density of 12T than environment without magnetic field.