Abstract: The vaccum brazing tests of 20 Steel and GH3030 alloy dissimilar metals were carried out by using pure Cu brazing alloy under different brazing gaps. The results show that when the brazing gap is 0.5-0.8mm,partially incomplete penetration defects appear in the longitudinal direction of the brazed joint, and there is no obvious reaction layer between brazing seam and 20 steel/GH3030 alloy. When the brazing gap is 1.0-1.3 mm,the brazing joint is compact and complete,without macroscopic defects in the longitudinal direction, and reaction layers are formed obviously between the brazing seam and 20 steel/GH3030 alloy. When the brazing gap is 1.5-1.8 mm, seriously incomplete penetration defects appear in the longitudinal direction of the brazed joint. The matrix of the brazing seam is Cu-Ni solid solution, and there are a small amount of Fe and Cr elements. When the brazing gap is 1.0-1.3 mm,a small amount of Fe-based solid solution is observed in the Cu-Ni solid solution matrix.