Defect modulation currently plays a decisive role in addressing the poor photoabsorption, sluggish electron hole separation, and high CO2 activation barrier in photocatalytic CO2 reduction. However, hunting for a straightforward strategy to balance the concentration of oxygen vacancy and metal cation defect in one photocatalyst is still a great challenge. Herein, a bismuth vacancies BiOBr nanosheets (BiOBr-1) on the exposed [001] facets were constructed via an acetic acid molecule modification strategy, which can repair oxygen defect by bismuth vacancy in low-temperature solid-state chemical method. Benefiting from the formed bismuth defects that can not only broaden light absorption and elevate charge separation efficiency, but also enhance adsorption and activation of CO2 molecules, the evolution rates of photocatalytic CO2 conversion into CO (71.23 µmol·g−1·h−1) and CH4 (8.90 µmol·g−1·h−1) attained by BiOBr-1 are superior 7.1 and 11 times to that of plate-like BiOBr. The photocatalytic mechanisms including adsorption concentration and activation process of CO2 are further revealed by the in situ diffuse reflectance infrared flourier transform spectra (DRIFTS). This finding of the existence of distinct defects in ultrathin nanosheets undoubtedly leads to new possibilities for photocatalyst design using two-dimensional materials with high solar-driven photocatalytic activity.