Photocatalytic degradation has been unearthed as a promising strategy for environmental remediation, and the calling is endless for more efficient photocatalytic system. In this study, a novel oxygen vacancy-rich two-dimensional/two-dimensional (2D/2D) BiOCl-g-C3N4 ultrathin heterostructure nanosheet (CN-BC) is successfully prepared by a facile solvothermal method for degradation of non-dye organic contaminants. HRTEM observes the formation of heterojunction, while ESR and)CPS unveil the distinct oxygen vacancy concentrations. Density functional calculations reveal that the introduction of oxygen vacancies (OVs) brings a new defect level, resulting in the increased photoabsorption. Under visible light irradiation, the OVs-rich optimum ratio of CN-BC (50CN-50BC) Exhibits 95% removal efficiency of 4-chlorophenol within 2 h, which is about 12.5, 5.3 and 3.4 times as that of pure BiOCl, g-C3N4 and OVs-poor heterostructure, respectively. The photocatalytic mechanism of OVs-rich 50CN-50BC is also revealed, suggesting that the synergistic effect between 2D/2D heterojunction and oxygen vacancies greatly promotes visible-light photoabsorption and photoinduced carrier separation efficiency with a prolonged lifetime, which is confirmed by multiple optical and electrochemical analyses, including DRS, steady-state photoluminescence spectra, electrochemical impedance spectroscopy, photocurrent response and time-resolved fluorescence spectra. This study could bring new opportunities for the rational design of highly efficient photocatalysts by combining 2D/2D heterojunctions with oxygen vacancies in environmental re mediation.