Bismuth-based compounds have been regarded as an important class of visible-light photocatalysts due to their special electronic structures. In this paper, iodide ions are introduced to modify bismuth-based compound, Bi24O31Br10, forming a Bi24O31Br10/BiOI heterojunction structure. A significant enhancement of photocatalytic activity compared to the parent compounds is observed in de-coloration of rhodamine B (Rh.B) solution. The improved photocatalytic property of Bi24O31Br10/BiOI heterojunction is ascribed to the unique electronic structure consisting of complementary band structures of BiOI and Bi24O31Br10. Iodide ions are regarded as an effective reagent to construct bismuth-based photocatalytic heterojunctions with improved photocatalytic activity.