The conventional p-n heterojunction photocatalysts suffer from the incompatibility between the interfacial charge transfer efficiency and the redox ability of charge carriers. To optimize the interfacial charge transfer of the conventional BiOI/TiO2 p-n photocatalyst, we synthesized the BiOI/Bi/TiO2 ternary photocatalyst with sandwiched metallic bismuth (Bi0) by the oxygen-vacancy assisted method. The DFT calculation and structural characterizations confirmed the reaction of the electron-rich oxygen vacancies in the 2D-TiO2 nanosheets (TiO2-NS) with the adsorbed BiO+ species. This reaction broke the Bi-O bonds to form Bi0 nanoparticles in-situ at the interface but still maintained the p-n heterojunction well. The NO-TPD and XRD analyses for samples correlated the Bi0 formation with the oxygen vacancy concentrations well. The sandwiched Bi0 functioned as an electronic transfer mediator like that in the Z-scheme heterostructure. Comparing with 0.20 BiOI/TiO2-NP (NP=Nanoparticles), 0.20 BiOI/Bi/TiO2-NS-a (NS=Nanosheet) showed a much improved catalytic performance, i.e., duplicated apparent quantum yield (AQY) and triplicated reaction rate constant (k). Also, the formation mechanism and the reaction mechanism were investigated in detail. This work provides a new strategy for the improving of the conventional p-n photocatalysts and new insights into the nature of the photocatalysis.