Ceria (CeO2) with abundant oxygen defects, surface alkalinity, low cost effectiveness and admirable redox ability could be used in the photoreduction of CO2. However, little attention has been paid to the interaction of reactant CO2 molecules on CeO2-based photocatalysts. In this work, Cu2O nanoparticles were applied to the modification of the properties of Lewis acid/base, surface oxygen defect content and visible light adsorption of CeO2, and the adsorption/activation abilities of CO2 reactant on Cu2O/CeO2 and CeO2 photocatalysts were investigated in comparison. The photocatalytic performance showed that Cu2O/CeO2 had better activity than CeO2. And the loading of Cu2O resulted in more oxygen defects and Ce3+ species, which was helpful for available visible light adsorption and higher charge-separation efficiency. Furthermore, CO2-TPD, CO2-adsorption DRIFTS and in-situ ESR results demonstrated that the synergistic effects of Cu2O/CeO2 were beneficial for more generated carboxylate and (CO2-)-C-center dot radicals, instead of carbonate species which promoted CO2 reduction to CO.