To address the deterioration of the joint performance owing to the formation of brittle Mg-Al compounds, this paper proposes a laser fusion welding technology by adding Ni interlayer to magnesium and aluminum alloys'' interface layer. A lap joint is applied with a magnesium alloy sheet on the top and an aluminum alloy sheet at the bottom. The interlayer element is selected based on the calculations of thermodynamic properties and mechanical properties calculations. Finally, the effects of Ni interlayer on the microstructure and properties of fusion welded joints are studied. Results show that the surface of the joint with an Ni interlayer exhibits a regular fish scale, the weld has good formability, and the maximum shear strength (linear load) of the joint is 78.92 N/mm under laser power of 1450 W, a welding speed of 1200 mm/min, a laser head deflection angle of 20°, a defocus amount of 0 mm, and side blowing and back protection using the Ar protective gas. The Ni interlayer foil acts as a barrier to limit the upward diffusion of the Al element in the lower aluminum alloy liquid into the upper magnesium alloy molten pool; therefore, the formation of brittle Mg-Al compounds is reduced. In addition, the reaction of Ni and Al in the molten pool region of alloys joint to form ductile AlNi and Al3Ni compounds, AlNi and Al3Ni compounds have better ductility, and their structures are more stable than those of Mg-Al compounds, implying that the joint brittleness is reduced. The above synergistic effect is an important reason why adding Ni interlayer improves the shear performance of welded joints of magnesium/aluminum alloys.