Abstract: Sandwich composite filler metal is an important technical means to realize reliable brazing connection between cemented carbide and steel. The microstructure of the composite layer and the interface layer between the composite layer and the matrix has an important influence on the mechanical properties. The microstructure evolution of brazing joint of gradient sandwich composite filler metal under different brazing temperature and time conditions was systematically studied, and the key factors affecting the mechanical properties were analyzed. The research results showed that the interfacial formation of gradient sandwich filler metal with steel and carbide matrix could be divided into four stages: interfacial structure formation, growth, fusion and rearrangement. The CuMn2 layer in the middle of the gradient filler metal gradually changed from the initial continuous shape to an isolated island surrounded by low Mn copper solid solution with the extension of holding time. As the holding time continued, CuMn2 layer disappeared and the whole brazing joint was composed of copper solid solution, silver solid solution and silver copper eutectic. The shear strength of the brazing joint reached the highest 285 MPa when the temperature was kept at 780 ℃ for 2.5 min. At this time, the Co and Ni elements had long range diffusion and accumulated near the CuMn2 interlayer, which improved the strength and toughness of the CuMn2 interlayer. The Co-based particle strengthening phase was distributed in the root of the fracture dimple. Further extension of holding time, Co, Ni and other brazing elements began to disperse, the brazing structure coarsened, the strength decreased.