Oxide defect engineering in semiconductors is of growing interest and considered as an important strategy for promoting photocatalytic performance, as it enables to couple solar energy into oxygen activation. Herein, the surface oxygen vacancies are introduced into bismuth-rich Bi4O5I2 nanosheets (Bi4O5I2-OV) via a facile solvothermal route. By introducing oxygen vacancies into Bi4O5I2, the molecule oxygen adsorption is significantly improved. More importantly, attributed to the molecular oxygen adsorption, an instantaneous surface-bulk homojunction is constructed, which facilitates the separation and transport of photogenerated charge carriers. Therefore, noticeable molecular oxygen activation and N-2 fixation are achieved in Bi4O5I2-OV. These findings may shed light on designing highly efficient photocatalysts and provide fresh insights into understanding the charge migration mechanism in oxygen vacancies-related photocatalytic system.