Cu is widely used to prepare high-value products in photocatalytic CO2 reduction reaction (CO2PR). The valence state of transition metals usually has a great impact on the catalytic process. However, the research on Cu valence in CO2PR is lack for its complex valence change. In current work, Cux/P25 with stable Cu valence composition in CO2PR is prepared. The results show that Cu is composed of Cu0 and Cu2O, and their proportions change regularly as Cu loading changing, which are linearly related to the selectivity of the corresponding products of CO2PR. Combined with thermodynamic and kinetic analysis, the CO adsorption and activation on the surface of Cu0 and Cu2O are considered to be the key to determine the product of CO2PR, which is further confirmed by DFT calculation. Cu0 and Cu2O are proved to be active site of producing CH4 and CO, respectively. In this case, a reference for the study of highly selectivity Cu based photocatalysts is provided.