Abstract: The heteroepitaxial growth of high-quality β-Ga₂O₃ remains constrained due to lattice mismatch and interfacial stress. Here, we demonstrate high-quality β-Ga₂O₃ film grown on sapphire substrates via plasma-assisted molecular beam epitaxy using a multilayer transition strategy that incorporates an AlN/GaN buffer layer and a GaON nucleation layer. First-principles calculations reveal that GaON serves as an effective nucleation layer for the β-Ga₂O₃ epitaxy, due to its matched N–Ga–O bond angle (90.27°) with the GaO₆ octahedra of β-Ga₂O₃ and a 50% lower effective plane energy (0.109 eV Å⁻²) than sapphire. The reduction in the rocking curve full width at half maximum from 7063 to 4341 arcsec indicates a substantial improvement in the crystal quality of the β-Ga₂O₃ film and clear high-resolution transmission electron microscopy imaging revealing distinct atomic periodicity across the heterointerfaces. Energy band diagram analyses further indicate that the GaON effectively reduces the band offsets and suppresses interfacial recombination. Impressively, the resulting ultraviolet photodetectors exhibit excellent performance, with a dark current as low as 13 pA, a photo-to-dark current ratio of 6.29 × 10⁶, a responsivity of 3821.6 A W⁻¹, and a specific detectivity of 3.3 × 10¹⁶ Jones, alongside a fast response speed (rise/decay times of 30/20 ms). This work introduces a bond angle–mediated nucleation strategy that overcomes fundamental lattice-mismatch constraints in Ga₂O₃ heteroepitaxy, establishing a novel paradigm for growing high-quality semiconductors.