Abstract: Based on the cold spray technology, the Cu coating was prepared on the surface of the stainless steel substrate as an interlayer in advance, then connected to the Al alloy by resistance thermal diffusion welding. The influence of the thickness of the Cu coating on the interface structure and mechanical properties of the final joint was discussed utilizing relevant analysis methods. The results indicated that a good metallurgical bond was formed between the pre-prepared Cu coating and the stainless steel substrate by cold spray technology. In the subsequent resistance thermal diffusion welding process, the Cu coating acted as an interlayer to effectively prevent the interdiffusion of Al and Fe elements, which was attributed to the interlayer converting the original Al-steel connection into Al-Cu connection. When the Cu coating thickness was 100 μm, a defect-free reaction layer was generated between the Al alloy and the Cu coating. The main component was Al(s.s) and a small amount of Al₂Cu in the reaction layer. The tensile shear load of the joint reached 755.39 N, and the fracture position was located at the reaction layer. However, the tensile shear load of the joint gradually decreased due to the increase of the Cu coating thickness because the increased Cu coating thickness increased the joint resistance, forming a higher temperature field in the welding process, which led to welding defects such as cracks in the reaction layer due to the formation of more Al-Cu intermetallic compounds.