Abstract: The diffusion treatment after vacuum brazing is an important technique to improve the quality of brazing joint and eliminate the defects of joint. Cu-P based filler was carried out by studying effects of diffusion treatment after vacuum brazing using a tube-sheet joint, and the influence of diffusion treatment on brazing joint was investigated by means of electron microscope, energy spectrum analysis and tensile force test. The results showed that the brazing joint structure was composed by copper-based solid solution and copper-phosphorus binary eutectic, and the brazing seam center interface areas became evidently more narrowed after brazing diffusion treatment, which makes brazing organization more and more uniformly. Meanwhile, small amounts of phosphorus can refine the copper grain; as a result, the brittleness of Cu₃P decreased and copper phosphorus binary eutectic was gradually reduced. Tensile force test found that tensile strength was up to 171 MPa on diffusion-treated than 143 MPa with non-diffusion, and the plasticity became better.