Abstract: Resistance spot welding (RSW) was conducted on 2.8-mm-thick 7075 aluminum alloy by adding a 50-μm-thick pure Mg interlayer to study the effect of Mg interlayer on the microstructure and properties of spot welding joints. The macroscopic morphology, microstructure, precipitation phase, microhardness, and mechanical properties of the RSW joints before and after the addition of the Mg interlayer were compared and studied. The results show that after adding an Mg interlayer, the welding quality and mechanical properties of 7075 aluminum alloy’s RSW joints are significantly improved. The Mg element content in the center zone of the weld nugget is significantly increased, and the grain size in the equiaxed crystal region is significantly refined. The optimal welding parameters for the process of adding the Mg interlayer are a welding current of 34 kA, a welding time of 0.36 s, and an electrode pressure of 5.5 kN. At this point, the average grain size in the equiaxed crystal region decreases from 31.4 μm without the addition of the Mg interlayer to 23.2 μm, a reduction of 26.1%. The precipitation phase changes from long stripes to dots and short stripes; the number increases; the distribution is more uniform. Defects such as shrinkage cavity (SC) and cracks in the joint are significantly reduced. The average microhardness in the center region of the weld nugget increases from 103.1 HV to 115.3 HV, an increase of 11.8%. The maximum tensile-shear force of the joint increases from 8.02 kN to 9.1 kN, an increase of 13.5%.