Heterojunction catalysts have been demonstrated to significantly enhance photocatalytic CO₂ reduction activity. However, the direction and rate of charge transfer at the catalyst interfaces remain the primary limiting factors for catalytic performance. In this study, a ternary heterojunction MgTi₂O₅ (MT)/TiO₂/g-C₃N₄ (CN) was prepared via in situ composite synthesis, and rare earth single-atom Ho, with abundant 4f electron energy levels, was utilized as an electron transfer channel within the heterojunction. This approach effectively facilitated electron transfer at the catalyst interface, and its photocatalytic activity and mechanism were thoroughly investigated. Remarkably, the catalyst exhibited distinct charge transport mechanisms under visible light and ultraviolet light. In the absence of a sacrificial agent, the sample MT:CN-1 demonstrated optimal performance under visible light, achieving a CO yield of 31.42 μmol·g⁻¹·h⁻¹, which is 9.8 times higher than that of pristine CN. Under ultraviolet light, the sample MT:CN-2 exhibited the best performance, with a CO yield of 40.61 μmol·g⁻¹·h⁻¹, representing a 4.2-fold enhancement compared to CN. Furthermore, the charge transfer mechanism of the catalyst was elucidated using in situ X-ray photoelectron spectroscopy (XPS) and femtosecond transient fluorescence spectroscopy techniques.