The vacancy defects of semiconductor photocatalysts play key roles in enhancing their photocatalytic CO2 reduction activity. In this work, CeO2 was chosen as a model catalyst and oxygen vacancies were introduced on its surface by a facile and mild oxalic acid treatment followed by moderate heating in N2. Such a treatment resulted in a much increased ratio of Ce3+/Ce4+ in CeO2, and the oxygen vacancy-enriched CeO2 showed remarkably enhanced photocatalytic activity in CO2 reduction, with CO being the dominant reduction product, whose yield was about 8 times that on the pristine CeO2. In situ FT-IR spectra showed that the abundant oxygen vacancies substantially improved the CO2 adsorption/activation on the surface of CeO2, which facilitated the subsequent reduction of CO2. However, the carbonates strongly adsorbed on the photocatalyst surface might be the main obstacle to maintaining the high CO2 reduction activity and stability of CeO2 with O vacancies.