The transient liquid-phase (TLP) diffusion bonding of GH5188 with a BNi-5 interlayer was focused on. Parameters were chosen and optimized for GH5188 alloy according to the TLP joining mechanism. The microstructure evolution and mechanical properties of the joints were studied. Results show that the relatively complete isothermal solidification zone (ISZ) ensures a reliable connection of the base metal (BM). Within the temperature range of 1110–1190 °C, higher bonding temperatures can widen ISZ and promote joint composition homogenization, thus improving mechanical properties. However, the increase in precipitated phase has an adverse effect on the mechanical properties of the joint. The maximum shear strength, reaching 482 MPa, is achieved at 1130 °C, representing 84.6% of BM strength. Within the pressure range of 5–15 MPa, both precipitated phases in adiabatic solidification zone (ASZ) and voids generated by partial melting increase. On the contrary, their sizes decrease significantly under higher bonding pressure, resulting in an upward trend in alloy mechanical properties. The maximum shear strength of 490 MPa is attained at a bonding pressure of 15 MPa. The joint exhibits a typical mixed fracture pattern, with the small brittle M₂₃C₆ phase and voids significantly impacting mechanical properties. Nano-indentation tests indicate that ASZ is a potential source of cracks.