Heterostructured photocatalysts are promising candidates in the photocatalysis field, and the heterojunction plays a vital role in the separation of spatial charge carriers. Here, a heterojunction was fabricated by the in situ growth of ultrathin Bi12O17Cl2 (BOC) nanosheets (NSs) onto porous g-C3N4 (PGCN) NSs. The NSs' nanostructure can effectively shorten the diffusion path of charge carriers and thus promote interfacial charge migration, which can improve the surface photocatalytic activity. The X-ray photoelectron spectroscopy spectra and the experimental measured Fermi level (E-F) indicate that electrons transfer from PGCN to BOC, which leads to the formation of the built-in electric field with the orientation from PGCN to BOC. Driven by the built-in electric field, the charge carriers transfer through a step-like pathway. This step-scheme porous g-C3N4/Bi12O17Cl2 (PGCN/BOC) heterostructured nanocomposite displays an enhanced photocatalytic performance compared with pure BOC and PGCN. This work provides new insight into the novel construction of a step-scheme heterojunction toward photocatalytic CO2 reduction. (C) 2020 Elsevier Inc. All rights reserved.