Due to the complementary advantages of structure and performance,copper-stainless steel pipe composite components play an important role in the cooling system of nuclear reactor equipment. In view of the complex and difficult vacuum diffusion welding process of Cu-stainless steel in the nuclear island cooling system,the welding process is designed and the formulation of the performance study scheme,the performance evaluation and microstructure observation are carried out,which is helpful to solve the existing problems in welding. In this experiment,the method of adding Ni as the intermediate layer of coating is adopted. The vacuum diffusion welding of copper and stainless steel is carried out at a certain temperature,pressure and vacuum degree. The mechanical properties of the cop-stainless steel joint are evaluated by shear test and hardness measurement. The microstructure and diffusion bonding process of the joint are analyzed by ZEISS microscope,SEM scanning electron microscope and EDS. Through the random sampling analysis and comparison of the welded joints,it is found that the welded joints obtained by the diffusion welding test still have defects such as insufficient joint and unbalanced performance,and the welding process needs to be further optimized.