The work aims to improve the weldability of diamond and facilitate the bonding between the diamond and heterogeneous alloy. Ta coating was deposited on chemical vapor deposition (CVD) single-crystal diamond (not powder or particle) surface with large-size via double glow plasma surface alloying (DGPSA) technique. Then, the Ta-coated single-crystal diamond and cemented carbide (WC-Co) were brazed with Ag-Cu-2wt.%Ti brazing alloy at 810 ℃ for 10 min in vacuum furnace. The phase composition, surface micro morphology and element distribution of Ta coating and the cross-sectional morphology after brazing were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectrum (EDS). Furthermore, to verify the effectively weldability of Ta coating for single-crystal diamond, a universal testing machine was used to conduct a shear fracture test on the brazing samples of single-crystal diamond with Ta coating and without Ta coating (control group) to explore the interfacial bonding strength after brazing. After alloying at 850 ℃, the thickness of Ta coating increased gradually from 0.35 μm to 7.96 μm for deposition of 5, 15, 30 and 60 min, the grain grew from the nano-crystal to columnar crystal, and the average grain size increased from ~84.24 nm to ~136.11 nm according to the XRD pattern and Scherrer‘s equation with the deposition time at the same temperature of alloying (850 ℃). The surface of Ta coating sags and crests increased with the increase of deposition time and this surface unevenly improved the flow of brazing alloy and enhanced the adhesion between Ta-coated single-crystal diamond and WC-Co during brazing. Besides, combined with the SEM of cross-sectional morphology and EDS, the Ta coating was composed of deposited layer I and diffused layer Ⅱ. This diffused layer was mainly composed of TaC and Ta2C. These two metal carbides with good mechanical properties were generated at the single-crystal diamond/Ta coating interface. The Ta2C had mechanical properties between Ta and TaC, which made it possible to improve the interface plasticity and impact resistance of single-crystal diamond/Ta coating. In addition, this carbides layer played a vital transition function at the interface between the single-crystal diamond and the Ta coating. The well bonding was provided at this interface by this layer during shearing test. After shearing test, the shear strength first increased and then decreased as the deposition time of Ta coating increased. When the deposition time is 30 min, the thickness of Ta coating is 3.47 μm. The maximum shear strength (115.6 MPa) after brazing with WC-Co is obtained, which is greater than the shear strength (75.6 MPa) of brazing samples without Ta coating. It is proved that Ta coating can obviously promote the weldability of single-crystal diamond. It is worth noting that the sample with the longest deposition time (60 min) and the largest Ta coating thickness (7.96 μm) do not have the greatest shear strength.