Excessive CO2 emissions have led to serious environmental problems. The photocatalytic reduction of CO2 to value-added chemicals is a promising strategy to reduce carbon emissions and alleviate the energy crisis simultaneously. Photocatalysts is crucial in the reduction process. Nanostructure engineering and heterojunction construction have been identified as prospective approaches to develop efficient photocatalysts for CO2 reduction. Step-scheme (S-scheme) heterojunctions are novel systems composed of a reduction catalyst and an oxidation catalyst. In these systems, the charge separation at the interface between the two catalysts could be enhanced by an internal electric field directed from the reduction photocatalyst to the oxidation photocatalyst on account of their matched Fermi levels (Ef). The S-scheme transfer mode can not only efficaciously inhibit the recombination of photoinduced carriers but also accumulate electrons and holes with greater redox potential. Cu2O and BiOI materials, as typical reduction and oxidation catalysts, are endowed with efficient visible-light absorption and favorable band position for catalyzing the coupling reaction of CO2 reduction and H2O oxidation. In this study, a series of S-scheme catalysts consisting of polyhedral Cu2O-modified BiOI flakes were synthesized onto a fluorine-doped tin oxide substrate via the electrodeposition method. The structure, morphology, and surface composition of the as-obtained samples were then studied using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements. 13C/18O isotope tracer experiments indicated that the BiOI/Cu2O composite achieved CO2 conversion with water vapor under visible-light irradiation (λ> 400 nm). The CO, CH4, H2, and O2 yields of the optimal BiOI/Cu2O-1500 catalyst reached 53.03, 30.75, 8.49, and 82.73 μmol‧ m− 2, respectively, after 11 h of visible-light illumination. The photocatalytic activity of BiOI/Cu2O-1500 slightly decreased at the eighth cycling, but its CO, CH4, and O2 yields still reached 27.38, 34.08, and 75.52 μmol‧ m− 2, respectively. The XPS and XRD results confirmed the excellent cycling stability of the catalysts, and analysis using the XPS core-level (CL) alignment method revealed that a staggered band structure was formed in the BiOI/Cu2O heterojunction. The direction of the built-in electric field in the heterojunction was determined using UPS measurements, and the S-scheme mechanism of charge transfer was verified via the in situ XPS results. In addition, the production of HCO− 3, CO3 2−, HCOO−, and• CH3 species during CO2 reduction was confirmed using in situ diffuse reflectance Fourier transform spectrometry, and a possible mechanism of CO2 conversion under water vapor was proposed. Benefiting from its S-scheme BiOI/Cu2O heterojunction, the prepared catalyst showed improved photoinduced charge separation, and its photogenerated carriers with strong redox ability were preserved, thereby leading to enhanced photocatalytic performance.