Novel Bi₂S₃/BiOI heterostructures were successfully synthesized through a facile and economical ion exchange method between BiOI and thioacetamide (CH₃CSNH₂), and characterized by multiform techniques, such as XRD, Raman, FT-IR, XPS, SEM, TEM, HRTEM, SAED, BET and DRS. The obtained Bi₂S₃/BiOI photocatalysts showed excellent photocatalytic performance for decomposing organic dye methyl orange (MO) compared with pure BiOI under visible light irradiation (λ > 420 nm). Among the Bi₂S₃/BiOI photocatalysts with different molar percentage of Bi₂S₃ to initial BiOI (from 2 to 8%), 4% Bi₂S₃/BiOI exhibited the highest photocatalytic activity with apparent k_app of 0.2968 h⁻¹. Differently, Bi₂S₃/BiOI displayed low photocatalytic activity for many colorless organic substrates, such as phenol, 2-chlorophenol, dimethyl phthalate and 5-sulfosalicylic acid. Moreover, the study on the mechanism suggested that the enhanced photocatalytic activity mainly resulted from the role of Bi₂S₃–BiOI heterojunctions formed in the Bi₂S₃/BiOI, which could lead to efficient separation of photoinduced carriers.