The microstructure, micro zone properties, fracture morphology, and mechanical properties of different areas for the resistance spot welding joint of SUS301L stainless steel are systematically characterized via microhardness tester, electronic tensile shear testing machine, SEM, and IBIS. The results indicate that the morphology of the fusion zone in the joint is elliptical and which can be differentiated into the base metal zone, heat affected zone, and nugget zone. The microstructure distribution from the edge of the fusion core to the core center is columnar structure and equiaxed structure in sequence. The main defects of the joint include sputters on the edge of the fusion core, shrinkage porosity in the fusion core, and micro cracks, and the sputtering phenomenon is closely related to the occurrence of shrinkage porosity. The size of the nugget diameter is a key indicator that affects the mechanical properties, and there is a positive correlation between the two. As the thickness of the double-layer plate increases, the maximum tensile and shear force increment caused by the increment of the unit nugget diameter also significantly increases. The hardness and strength of the heat affected zone are lower than those of the base metal zone and the fusion zone, which leads to the formation of a softening zone at the corona bond. Due to the lower strength of this zone, the tensile shear force it can bear is relatively small, making it the starting area for tensile shear cracking. The forms of tensile shear fracture may be differentiated into passing through-core fracture and along-core fracture, the nugget diameter will affect the fracture mode of the joint.