Noble-metal co-catalysis could increase the CH₄ product selectivity for photocatalytic CO₂ reduction by TiO₂. However, the high cost and scarcity of noble metals are key obstacles for their extensive use. Thus, it is necessary to explore cheap alternatives. Herein, Mo-doped TiO₂ with different Mo concentrations were successfully prepared via a one-pot hydrothermal method at 473 K. The activity measurements of the photocatalysts show that the CH₄ selectivity is increased with the Mo concentrations increasing and reaches 54.1 % at a Mo concentration of about 0.3 wt%. With further increasing the Mo concentrations, the CH₄ selectivity begins to decrease. Transient photocurrent and fluorescence emission spectroscopy measurements, IR spectra for adsorbed D₂O and ·OH trapping suggest that the promoted electron-hole separation and proton supply are together responsible for the enhanced CH₄ selectivity. Moreover, photocatalytic CO, formaldehyde, and methanol reductions with H₂O and ·CH₃ trapping show that the enhanced CH₄ selectivity is not directly related with the interaction of CO₂ with catalyst surfaces. These reaction results also imply that photocatalytic CO₂ reduction might follow the fast-deoxygenation pathway over TiO₂-based catalysts. The present work supplies a guide to explore cheap modifiers for selective photocatalytic CO₂ reduction to CH₄.