An efficient reduced graphene oxide (rGO)-based cadmium sulfide/reduced graphene oxide/bismuth oxyiodide (CdS/rGO/BiOI) Z-scheme system was fabricated by an in-situ growth method. Solid interactions between the rGO and nanoparticles promoted the transmission of photoinduced carriers, which separated and accumulated electrons from CdS and holes from BiOI. Moreover, the Z-scheme system enabled CdS electrons to maintain a high reduction capability and BiOI holes to maintain a high oxidation capability. Therefore, the CdS/rGO/BiOI composite exhibited better photocatalytic activity in the degradation of methyl 3,5-dichloro-4-hydroxybenzoate (MDHB) than that of pure CdS and BiOI under visible light irradiation. In particularly, the identification of active species and transformation products revealed that the accumulated photoinduced electrons promoted the reductive dechlorination of MDHB. The dechlorinated byproducts were then oxidatively split and degraded into harmless small-molecule carboxylic acids or inorganic substances by photoinduced holes. This work provided a favorable in situ growth method to design and synthesize Z-scheme photocatalysts for simultaneous chlorinated paraben dechlorination and degradation under visible light irradiation.