3D hierarchical Bi@BiOCl microspheres were successfully synthesized by a facile solvothermal method using citric acid as a modulating agent and the growth process was revealed. The modulation of citric acid not only reduced the size of BiOCl nanosheets, finally transforming BiOCl microflowers into microspheres, but also induced the in situ reductive deposition of metallic Bi on the surface of the microspheres. Consequently, Bi@BiOCl microspheres showed larger specific surface areas and total pore volumes, higher absorptivity to the visible light and better charge transfer ability than BiOCl microflowers. As a result, Bi@BiOCl microspheres exhibited much better photocatalytic performance than BiOCl microflowers. Bi@BiOCl microspheres modulated by 2.8g citric acid showed the highest photocatalytic activity, which was 4.4 and 2.5 times higher than BiOCl microflowers in degrading RhB under visible light and salicylic acid under UV light, respectively. This work may provide a new insight into simultaneous size control and in situ metal deposition for Bi-containing photocatalysts and other materials.