Step‐scheme (S‐scheme) heterojunctions have been extensively studied in photocatalytic carbon dioxide (CO2) reduction due to their excellent charge separation and high redox ability. The built‐in electric field at the interface of a S‐scheme heterojunction serves as the driving force for charge transfer, however, the poor interfacial contact greatly restricts the carrier migration rate. Herein, we synthesized the g‐C3N4/Bi19Br3S27 S‐scheme heterostructure through in situ deposition of Bi19Br3S27 (BBS) on porous g‐C3N4 (P‐CN) nanosheets. The C−S bonds formed at the interface help to enhance the built‐in electric field, thereby promoting the charge transfer and separation. As a result, the CO2 reduction reaction performance of 10 %Bi19Br3S27/g‐C3N4 (BBS/P‐CN) reaches 32.78 μmol g−1h−1, which is 341.4 and 18.7 times higher than that of pure BBS and P‐CN, respectively. X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) prove the presence of chemical bonds (C−S) between the P‐CN and BBS. The S‐scheme charge‐transfer mechanism was analyzed via XPS and density functional theory (DFT) calculations. This work provides a new idea for designing heterojunction photocatalysts with interfacial chemical bonds to achieve high charge‐transfer and catalytic activity. The effective construction of interfacial C−S bond and built‐in electric field in g‐C3N4/Bi19Br3S27 S‐scheme heterojunction improves the separation of photogenerated electrons and holes, resulting excellent CO2 reduction performance.