The implementation of Ni-based brazing filler metal for bonding SiC to metals can effectively expand the high-temperature applications of SiC-based composite materials. To reduce the excessive brittle phases formed in the reaction between SiC/Ni, this study used Au, a chemically stable element, as an additive to investigate the experimental mechanism of Au regulating the reaction between Ni and SiC. The study found that under certain temperature and reaction time conditions, Au can hinder the diffusion of Ni into SiC while suppressing the reaction between Ni and SiC at the interface. The microstructure evolution of the SiC-Nb joint brazed with an AuNi alloy was investigated, and the theoretical analysis was verified. The optimized mechanical strength reached 30 MPa. Based on experimental and theoretical analysis, the weakness area of the joint was identified and the formation mechanism was analyzed. Importantly, the interface behavior analysis obtained in this study can provide some new ideas for the optimization of alloy composition in brazing experiments.