This work presents a one-pot template-free synthesis, detailed characterization, growth mechanism and application of well-defined uniform monodisperse (BiO)2CO3 hierarchical hollow microspheres self-assembled with single-crystalline nanosheets. The synthesis was conducted by hydrothermal treatment of bismuth citrate and sodium carbonate in water. Time-dependent evolutions of phase structure, composition, and morphology were investigated systematically and revealed that the growth mechanism of such novel structures involved a unique multistep pathway. First, near amorphous particles were produced through reaction, nucleation, crystallization, and aggregation processes. Then, stacked embryos of intermediate (BiO)4CO3(OH)2 microspheres with attached particles were produced due to dissolution and recrystallization. Subsequently, stacked uniform solid microspheres with small particles attached on edges were generated by the consumption of particles through Ostwald ripening. The stacked microspheres further grew to form monodisperse hierarchical microspheres with a hole in the center, like flower buds. Finally, uniform monodisperse (BiO)2CO3 hierarchical hollow microspheres were produced through layers splitting. The aggregation of the self-assembled nanosheets contributed to the formation of 3D hierarchical architecture containing mesopores, which is favorable for efficient reactants transport and photo-energy harvesting. Furthermore, the band gap structure of (BiO)2CO3 was revealed by the experimental method combined with density functional theoretical calculation. As expected, the novel (BiO)2CO3 hierarchical hollow microspheres exhibited enhanced photocatalytic activity due to the special hierarchical morphology, exceeding that of (BiO)2CO3 particles and commercial P25. The as-prepared uniform (BiO)2CO3 microspheres with well-defined hierarchical hollow structures are also ideal candidates for investigating their architecture-dependent performances in other areas, such as solar energy conversion, catalysis, electronics and so on.