Ag-Cu-In-Ti low-temperature filler was used to braze the diamond and copper, and the effects of brazing temperature and soaking time on the microstructure and mechanical properties of the joints were investigated. In addition, the joint formation mechanism was discussed, and the correlation between joint microstructure and mechanical performance was established. Results show that adding appropriate amount of In into the filler can significantly reduce the filler melting point and enhance the wettability of filler on diamond. When the brazing temperature is 750 °C and the soaking time is 10 min, a uniformly dense braze seam with excellent metallurgical bonding can be obtained, and its average joint shear strength reaches 322 MPa. The lower brazing temperature can mitigate the risk of diamond graphitization and also reduce the residual stresses during joining.