In this study, a novel visible-light-driven Bi₄O₅Br₂ photocatalyst was successfully synthesized via the structure reorganization of BiOBr at room temperature using NH₃·H₂O as a structure-controlling agent. The obtained Bi₄O₅Br₂ exhibited outstanding visible light activity and stability compared to BiOBr and P25 for the degradation of resorcinol. The physicochemical properties of Bi₄O₅Br₂ were analyzed and calculated by modern characterization techniques and density functional theory (DFT). The results revealed that the excellent performance could be mainly attributed to the effect of O-richness on the electronic properties of Bi and Br atoms, unique morphology, high visible-light absorption capacity, and prominent oxidation ability of photo-induced holes. Radical trapping experiments demonstrated that h⁺ and ˙OH radicals were the dominant active species. Moreover, a structure reorganization mechanism was proposed, revealing that ammonia and the water-steeping process both played important roles in the fabrication of Bi₄O₅Br₂. We believe that this facile method could be extended to fabricate other three component Bi–O–Br nanostructure systems and help elucidate the relationship between BiOBr and Bi_xO_yBr_z.