A novel flower-like 3D ZnO superstructure with high uniformity was fabricated on a large scale through a facile aqueous solution route at room temperature. The as-prepared ZnO superstructures with a diameter of about 3 μm are assembled by large amounts of interleaving nanosheets, which have a uniform thickness of about 15 nm and a well-crystalline structure with {100} planes as basal facets. The hierarchical microstructures, which were formed via a two-stage nucleation–growth process, highly depend on the concentration of NaOH and trisodium citrate dihydrate. The photoluminescence result indicates that the ZnO superstructures possess a relatively strong UV emission. The photodecomposition of methyl orange indicates that such ZnO superstructures possess excellent photocatalytic activity.