Diamond/Cu components are ideal choices for controlled nuclear fusion microwave windows. However, diamond brazing poses numerous challenges due to its poor wettability and susceptibility to graphitization at high temperatures. In this work, Ag-Cu-In-Ti low-temperature filler alloy was used to braze the diamond and copper, and the effect of brazing temperature and soaking time on the microstructure and mechanical properties of the joints were investigated systematically. In addition, the joint formation mechanism was clarified, and the correlation between joint microstructure and mechanical performance was established. The results showed that adding an appropriate amount of In into the filler significantly reduced its melting point and enhanced the wettability of the filler toward the diamond. When brazed at 750 °C for 10 min, a uniformly dense braze seam with excellent metallurgical bonding was obtained, achieving an average joint shear strength of 322 MPa. The lower brazing temperature applied mitigated the risk of diamond graphitization and also reduced the residual stresses during joining. The findings obtained provide fundamental theoretical and technical guidance for the high-performance manufacturing of diamond microwave windows in fusion reactors.