Solar-driven CO₂ reduction holds great promise for sustainable energy, yet the role of atomic active sites in governing intermediate formation and conversion remains poorly understood. Herein, a synergistic strategy using Ni single atoms (SAs) and surface oxygen vacancies (O_v) is reported to regulate the CO₂ reduction pathway on the Bi₂WO₆ photocatalyst. Combining in-situ techniques and theoretical modeling, the reaction mechanism and the structure-activity relationship is elucidated. In-situ X-ray absorption spectroscopy identifies Bi and Ni as active sites, and in-situ diffuse reflectance infrared Fourier transform spectroscopy demonstrates that adsorption of H₂O and CO₂ readily forms CO₃²⁻ species on the O_v-rich catalyst. Optimally balancing Ni SAs and O_v lowers the energy barrier for the formation and dehydration of a key COOH intermediate, leading to favorable CO formation and desorption. Consequently, a superior CO production efficiency of 53.49 µmol g^‒1 is achieved, surpassing previous reports on Bi₂WO₆-based catalysts for gas-phase CO₂ photoreduction.