Photocontrolled composites are worth studying because they exhibit different types of heterojunctions and tunable characteristics under different wavelengths of light. Here, photocontrolled heterojunctions constructed from Ho single atom modified Mg_1.2Ti_1.8O₅/g-C₃N₄ exhibit different photocatalytic product selectivities and electron transfer directions under visible and ultraviolet light irradiation, indicating different photocatalytic mechanisms, both of which exhibit excellent photocatalytic performance. The results of XAFS, in situ XPS, and fs-TAS indicated that Ho coordinated with N and O, and preferentially combined with g-C₃N₄ to form more N vacancies, which was beneficial for charge transfer and CO₂ adsorption. The ⁵G₃(³H₆)/³K₇(⁵G₄)/⁵G₅/⁵G₆(⁵F₁) → ⁵I₈ emissions can be absorbed by Mg_1.2Ti_1.8O₅/g-C₃N₄ composites, improving the light capture capability. The synergistic effects of Ho and Mg_1.2Ti_1.8O₅/g-C₃N₄ composites can reduce the ΔG of the key rate-determining step CO₂* → COOH*.