Abstract: To meet the demand for reliable joining of optical window structural components in the fields of high-precision optics, precision instruments, and aerospace, chemical vapor deposition (CVD) polycrystalline diamond was brazed to 4J333 expansion alloy using an Ag-Cu-In-Ti low-temperature filler metal, and sound brazed joints with defect-free interfaces were obtained. The effects of brazing temperatures of 700 ℃, 730 ℃, and 760 ℃ on the interfacial microstructure and mechanical properties of the joints were investigated. The results indicate that the brazed joint interface exhibits a typical layered structure. From the CVD polycrystalline diamond to the 4J333 expansion alloy, the structure successively consists of the diamond substrate, TiC interfacial reaction layer, intermediate brazing seam zone, alloy-side diffusion layer, and 4J333 substrate. The brazing seam zone is mainly composed of Ag(s,s), Cu(s,s), Ag₃In, Cu₇In₃, Ni₃Ti, (Fe,Ni)Ti, and Fe₂Ti; the diffusion layer adjacent to the 4J333 side contains (Fe,Co), Cu₇In₃, Ag(s,s), and Cu(s,s). As the brazing temperature increases, the amount of brittle compounds such as Ni₃Ti increases, their morphology coarsens, and they gradually migrate toward the polycrystalline diamond side; the dissolution zone of the 4J333 expansion alloy widens, and the thickness of the TiC layer on the polycrystalline diamond side increases. With the increase in brazing temperature, the shear strength of the joints first increases and then decreases. Under the process conditions of a brazing temperature of 730 ℃ and a holding time of 10 min, the average shear strength of the joints reaches the maximum value of 113.5 MPa. Fracture analysis indicates that the fractures of the joints all occur at the TiC interface and exhibit brittle fracture characteristics.