Photocatalytic reduction of CO2 into organic molecules is a very complicated and important reaction. Two possible pathways, the fast-hydrogenation (FH) path and the fast-deoxygenation (FdO) path, have been proposed on the most popular photocatalyst TiO2. We have carried out first-principles calculations to investigate both pathways on the perfect and defective anatase TiO2(101) surfaces to provide comprehensive understanding of the reaction mechanism. For the FH path, it is found that oxygen vacancy on defective surface can greatly lower the barrier of the deoxygenation processes, which makes it a more active site than the surface Ti. For the FdO path, our calculation suggests that it can not proceed on the perfect surface, nor can it proceed on the defective surface due to their unfavorable energetics. Based on the fact that the FH path can proceed both at the surface Ti site and the oxygen vacancy site, we have proposed a simple mechanism that is compatible with various experiments. It can properly rationalize the selectivity of the reaction and greatly simplify the picture of the reaction. The important role played by oxygen vacancy in the new mechanism is highlighted and a strategy for design of more efficient photocatalysts is proposed accordingly.