Abstract The band engineering is an efficient route to utilize solar energy to solve environment pollution with the help of visible-light-driven photocatalysts. In this work, two different narrow band gap semiconductors, porous g-C3N4 (Pg-C3N4) and Bi2S3, were prepared by one-pot method to form the step-scheme Bi2S3/Pg-C3N4 heterostructure photocatalyst. The crystal phase, morphology and optical properties of as-prepared samples were measured by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Vis DRS spectra. Bi2S3/Pg-C3N4 heterostructure exhibits excellent photocatalytic activity under visible light, which is much better than that of pure Bi2S3 and Pg-C3N4. The constant kapp of Bi2S3/Pg-C3N4 heterostructure is 0.01993 min−1, which is 16.33 and 29.74 times greater than that of pure Pg-C3N4 and Bi2S3, respectively. In addition, the photocatalytic degradation efficiency of Bi2S3/Pg-C3N4 heterostructure can remain 80.2% after three cycles. Finally, a step-scheme photocatalytic mechanism of Bi2S3/Pg-C3N4 was proposed.