The joints with Cu and Al as base materials were brazed by a new BAl88Si filler metal. By using the field emission scanning electron microscope, energy spectrum analyzer and transmission electron microscope (TEM), the threedimensional morphologies of intermetallic compounds in the brazed joint of filler metal were studied, and the lattice mismatch degree of each IMC interface was emphatically analyzed. The results show that the brazed joint of the filler metal has achieved good metallurgical bonding, and the interface micro structure of the Cu/BAl88Si/Al joint is Cu/Cu9Al4+CuAl2+CuAl2+ CuAl2-a-Al eutectic + α-Al/Al. Among them, Cu/Cu9Al4 has the best lattice matching, and it is a complete semi-coherent crystal plane. The interface of Cu9Al4/CuAl2 is a complete noncoherent crystal surface without. Based on the thermodynamic analysis, the formation and growth sequence of the intermetallic compounds in the brazing process was divided into four stages, and the formation of CuAl2 in the brazing seam was firstly confirmed by Gibbs free energy, followed by the formation of Cu9Al4.