As a representative fourth-generation ultra-wide-bandgap semiconductor, β-Ga₂O₃ has shown great promise for high-voltage, high-frequency, and high-power electronic devices because of its ultra-wide bandgap, high breakdown electric field, and low fabrication cost. However, its intrinsically low thermal conductivity makes it difficult to dissipate heat efficiently during device operation. This severely limits its performance and reliability under high power density. In recent years, heterogeneous bonding of β-Ga₂O₃ with high-thermal-conductivity materials such as diamond has emerged as an important strategy to overcome this thermal bottleneck. Such structures can potentially combine excellent electrical properties with efficient thermal management. However, the microscopic evolution of the β-Ga₂O₃/diamond interface and the crystal-orientation dependence of interfacial bonding strength remain unclear. This knowledge gap has hindered interface design, bonding process optimization, and reliability improvement. To address these issues, this study combined large-scale molecular dynamics simulations with density functional theory calculations. A million-atom training dataset with first-principles accuracy was constructed. Based on this dataset, a high-accuracy Moment Tensor Potential (MTP) was developed. Using the fitted potential, we systematically investigated the interfacial structural evolution of β-Ga₂O₃/diamond heterogeneous bonding models containing an amorphous transition layer during relaxation. We also examined how interfacial bonding strength depends on different crystal orientation combinations. The results show that, for all orientation combinations, the amorphous interlayer effectively relieves lattice mismatch and local stress concentration during relaxation. As a result, atomically dense contact is achieved at the bonded interface without obvious defects such as voids, cracks, or delamination. These findings indicate that the introduction of an amorphous interlayer improves interfacial structural integrity and bonding stability. Further mechanical analysis shows that the β-Ga₂O₃(010)/diamond(100) configuration exhibits the best interfacial bonding strength and overall mechanical performance, demonstrating a clear orientation-matching advantage. This work reveals, at the atomic scale, the evolution mechanism of β-Ga₂O₃/diamond heterogeneous bonding interfaces and the crystal-orientation dependence of their bonding strength. It provides important theoretical guidance for interface design, wafer bonding process optimization, and the improvement of mechanical stability and long-term reliability in high-power devices.