The facet-dependent photocatalytic activity of TiO2 has been found in CO2 photoreduction. However, which facet is more reactive and the nature of its dramatically different activity are still in the great debate. Utilizing density functional theory, two key photocatalytic steps (photo-generated electrons transfer and photoreaction) are investigated on anatase TiO2 (001) and (101) facets to unravel such puzzles. The photo-generated electrons transfer (within the bulk, from bulk to surface and across the surface) has similar transfer barrier, indicating that the experimentally observed different activity may not be dominated by photo-generated electrons transfer, and the photoreaction process could be the rate-limiting step. The possible CO2 photoreduction reaction paths have been extensively examined and large barrier difference on two facets is obtained. The facts that the higher conduction band minimum can generate more strongly reductive electrons, the lower reaction barrier can lead to the faster reaction rate and the product adsorption can have more suitable properties give rise to a higher activity on (101) facets than that on (001) facets. (C) 2016 Elsevier B.V. All rights reserved.