The photocatalytic performance of BiOI was limited by slow electron transfer and fast charge recombination. In this study, RGO/BiOI/ZnO (RBZ) composites were successfully synthesized by two-step hydrothermal method. The phase structure, morphology, and optical property of the as-synthesized samples were characterized by Xray diffraction, scanning electron microscopy, transmission electron microscopy, ultraviolet-visible diffuse reflectance spectroscopy and photoluminescence. The results revealed that BiOI appears as a flower-like micro spheres assembled by nanosheets which are anchored over RGO surface, while ZnO nanoparticles attached on the BiOI nanosheets. The photocatalytic activity of RBZ composites with different ZnO contents was investigated by the photocatalytic reduction of aqueous Cr(VI) under visible-light irradiation. The highest photocatalytic activity was observed in samples of RBZ composite with 8% ZnO. This can be attributed to the significantly enhanced efficient separation and transfer of photoinduced electron-hole pairs in RBZ composite. Therefore, the RGO/BiOI/ZnO composite photocatalyst can be a favorable alternative for practical use in environmental photocatalysis.