An efficient g-C3N4/Cu2O@Cu plasmonic Step-scheme (S-scheme) heterojunction photocatalyst was successfully designed and used for the degradation of pollutants and energy generation. The removal of Bisphenol-S pollutants and catalytic reduction of CO2 for hydrocarbon fuel production experiments under the visible light irradiation showed that the catalyst displayed good stability and perfect photocatalytic performance. Among the prepared samples, g-C3N4/Cu2O@Cu-4 displayed the highest catalytic performance, which was attributed to the high light absorption capacity and the efficient interfacial charge separation in the S-scheme heterojunction. From the viewpoint of practical wastewater treatment, a series of the effects of environmental factors, such as initial pollutant concentration, inorganic salts, organic compounds and various water sources on the photocatalytic performance were investigated. Eight intermediate products formed in the photocatalytic oxidation of Bisphenol-S were confirmed by the GC-MS, and the proposed photocatalytic degradation pathway of Bisphenol-S was suggested according to the intermediate products. Importantly, the charge density difference of the interface between g-C3N4 and Cu2O (g-C3N4/Cu2O) as well as the interface between Cu and Cu2O (Cu/Cu2O heterostructures) was calculated, respectively. The calculated results verified that the built-in electric field had been established at their interface. Spatial separation of photogenerated electron-hole pairs in the S-scheme g-C3N4/Cu2O@Cu heterojunction was realized through the built-in electric field. The charge density difference verified the internal electric field (E) had been established, and the spatial separation of photogenerated electron and holes followed Step-scheme heterojunction mechanism. Display omitted •g-C3N4/Cu2O@Cu was synthesized for pollutants degradation and energy generation.•A built-in electric field was fabricated in the g-C3N4/Cu2O@Cu.•The proposed photocatalytic degradation pathway of Bisphenol-S was suggested.•Comparative experiment and DFT study of g-C3N4/Cu2O@Cu were performed.