Solar-driven CO₂ methanation is an imperative and promising approach to relieve the global warming and environmental crisis, yet remains a great challenge due to the low reaction efficiency, unsatisfactory selectivity and poor stability. In this work, we demonstrate a facile and efficient strategy to prepare Ru-doped TiO₂ photocatalyst with tunable oxygen vacancies using the ascorbic acid as a reducing agent for the CO₂ methanation reaction. The optimal Ru-TiO₂-OV-50 exhibits a remarkable CH₄ production rate of 81.7 mmol g^-1 h^-1 with a 100% CH₄ selectivity under a 1.5 W cm^-2 light illumination, which is significantly higher than commercial Ru/TiO₂ and other reported catalysts. We reveal that the superior photocatalytic CO₂ methanation performance is mainly due to the synergistic effect of Ru doping and TiO₂ with tunable oxygen vacancies. Impressively, the light rather than thermal effect is confirmed as the main influencing factor by experimental studies. In addition, in situ spectroscopic technology is performed to investigate the CO₂ methanation reaction pathway. This work will open an avenue to design and prepare highly efficient photocatalyst with tunable oxygen vacancies for CO₂ conversion and other related applications.