Achieving high selectivity toward multi-electron products in photocatalytic CO₂ reduction remains a significant scientific challenge. In this study, we present the rational design of a novel heterojunction system comprising Zn/Fe/O co-doped CdS coupled with TiO₂ for efficient CO₂ photoreduction under simulated sunlight. The dual cationic (Ni and Fe) doping effectively modulates the band structure of CdS, introduces shallow trap states, and broadens visible-light absorption, while oxygen vacancies act as electron reservoirs that facilitate CO₂ adsorption and activation. Comprehensive characterization techniques, including HR-TEM, XPS, UV–Vis DRS, and BET analyses, confirm the successful incorporation of dopants, the presence of defect states, and the formation of intimate heterointerfaces conducive to interfacial charge transfer. The optimized photocatalyst, (Zn, Fe, O, Cd)S/TiO₂ (ZnFe₂O₄–CdS/TiO₂), exhibited the highest photocatalytic activity, achieving CO and CH₃OH production rates of 5.7 μmol·g⁻¹·h⁻¹ and 6.0 μmol·g⁻¹·h⁻¹, respectively—representing 1.8- and 1.7-fold enhancements compared to the pristine CdS/TiO₂ heterojunction. Photoluminescence analysis further revealed that the superior performance originates from the efficient spatial separation and transfer of photogenerated charge carriers induced by the synergistic effects of heterojunction formation and dopant–defect interactions. This study demonstrates an effective dopant–defect cooperative engineering strategy, providing valuable insights for the rational construction of advanced photocatalysts for solar-to-chemical energy conversion with remarkable methanol selectivity.