With the purpose of high-efficiency separation of photo-generated charges and an improved photocatalytic performance, herein we report novel heterojunctions of p-type BiOBr and n-type ZnO constructed by loading amounts of BiOBr nanoflakes onto the surface of ZnO nanoflowers via a two-step hydrothermal/solvothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) are employed to characterize the as-synthesized nanomaterials. The results indicate that p–n heterojunctions formed between BiOBr and ZnO. When evaluated as a photocatalyst for methyl orange (MO) decolorization, BiOBr–ZnO-40 shows excellent photocatalytic activity, superior to that of pristine BiOBr and ZnO. Notably, the photochemical process for the MO decolorization is elucidated by reactive species trapping and hydroxyl radical quantification experiments, suggesting that ˙OH and O₂˙⁻ are the main oxygen active species for BiOBr–ZnO heterojunctions in the MO decolorization. Moreover, the separation and transfer process of photo-generated charges at the surface or interface of the BiOBr–ZnO composites is also investigated by means of surface photovoltage (SPV). On the basis of experimental and theoretical results, the enhanced photocatalytic activity mechanism for the BiOBr–ZnO heterojunctions is proposed. The effective separation and transfer of photo-generated charges are responsible for an improved photocatalytic performance due to the existence of the interfacial electric field located between BiOBr and ZnO.