Phosphate modification is a general strategy for improving the photocatalytic activities of semiconductors. Ion exchange can be occurred during the phosphate modification process. This would give a surprise for a new phase formed on the surface of the parent photocatalyst. More importantly, this is conductive to form close contact and continuous interface favoring the charge transfer between two phases. Here, a novel BiOCl-OV-BiPO4 photocatalyst was fabricated using a facile in situ phosphate modification method. BiOCl-OV-BiPO4 displayed a maximum carbamazepine (CBZ) degradation efficiency of 81.70% after 30 min Xe lamp irradiation, with a reaction kinetics rate constant (k) of 5.75 times higher than ordinary BiOCl samples. Oxygen vacancy (OV) is important for the construction of Z-scheme BiOCl-OV-BiPO4: OV benefits the generation of BiPO4 on the surface of BiOCl; furthermore, OV acting as electron mediator, promotes the separation of photogenerated charge carriers between BiOCl and BiPO4. Based on the results from PL tests, photocurrent density, EIS Nyquist plots, and cyclic voltammetry, it can be demonstrated that Z-scheme structure of BiOCl-OV-BiPO4 not only increases the photogenerated electrons-holes separation rate but also retains their strong redox ability. This significantly facilitates the production of superoxide radical (O-2(-)) and holes (h(+)), which dominates the CBZ photodegradation. This work provides a new sight for the fabrication of Z-scheme photocatalysts to improve the photocatalytic performance.