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Photocatalytic CO2 reduction into hydrocarbon fuels has attracted considerable attention as an effective solution to solve the problems of energy crisis and global warming; however, the elementary reactions are complicated and the reactive mechanism is still indistinct. Herein, anatase TiO2 photocatalysts with tailored facets due to their generally excellent performance were chosen as model catalysts to investigate the underlying mechanism of CO2 reduction using the density functional theory. It was found that the CO2 reduction preferably occurred on the anatase {101} facet rather than on {010}. The rate-limiting step on the {101} facet was the final generation of CH4, while it was the initial reduction of CO2 on the {010} facet. Besides, the initial reduction of CO2 was a pivotal step and the photocatalytic efficiency and selectivity were distinctly promoted due to the regulation of the reaction processes over different facets. Using these results, we put forward a new approach to understand the relationship between the reaction process and the reaction rate in photocatalysis, which indicates that tailoring the rate-limiting step is a key to achieve excellent efficiency and product yield in energy- and environment-related catalytic reactions.