It is well established that the morphologies and sizes of photocatalysts have important effects on their photocatalytic activities; developing special nanostructures with excellent photocatalytic activity is a fundamental subject to promote the industrial application of photocatalysis technology. In this study, we have synthesized various Bi₂O₂CO₃ nanostructures via different hydrothermal routes, including nanosheets, nanoplates, nanoparticles, nanosheet-assembled hierarchical nanotubes, pinecone-like hierarchical nanostructures, and carnation flower-like hierarchical nanostructures. The properties of the as-synthesized nanostructures were investigated by various techniques. Their photocatalytic activities were systematically compared by degrading methylene blue (MB) and reducing Cr(VI) ions under simulated-sunlight irradiation, revealing that they exhibit largely different photocatalytic activities. Among various Bi₂O₂CO₃ nanostructures, the nanoparticles demonstrate the highest photodegradation activity for degrading MB, whereas the hierarchical nanotubes exhibit the highest photoreduction activity for reducing Cr(VI) ions. Moreover, the photodegradation of other organic pollutants over Bi₂O₂CO₃ nanoparticles was also investigated. The underlying photocatalytic mechanism was discussed and analyzed in details.