Under the tremendous pressure of imminent energy crisis and anthropogenic climate change, photocatalytic conversion of abandoned CO2 into energy-rich hydrocarbon fuels is highly desirable. However, this solar-to-fuel conversion is unavoidably suppressed by the fast recombination of electron-hole pairs and lack of stability of photocatalysts. To overcome this, we have developed ultrathin BiOBr nanosheets ( BOB-NS) with primarily exposed {001} facets. The {001} facets of BOB-NS are comprised of high density O atoms, which are linked to the neighbouring Bi atoms via weak BieO bonds with long bond length and low bond energy. After visible-lightdriven CO2 photoreduction over BOB-NS, we found that the BieO bonds were broken and oxygen vacancy ( OV) defects formed on the sample surface. The presence of these OVs was proven to be beneficial for photoactivities as photoinduced electrons were effectively trapped at the OV sites and recombination of charge carriers were inhibited. Generally, the free O atoms from dissociation of CO2 would reoxidize the sample surface, thereby deteriorating the performance of photocatalysts. In contrast, we demonstrated in this study that the OV sites on BOB-NS could be simultaneously regenerated and refreshed as the reactions proceeded, leading to a sustainable activity and long-term stability for CO2 photoreduction.