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In this work, we developed a facile, green and low-cost strategy for the controllable synthesis of birnessite-type MnO2 (Bir-MnO2) nanosheets assembled in a "desert rose"-like 3D hierarchical nanostructure and proposed a two-step growth route, namely photolysis and followed by auto-catalytic decomposition of KMnO4. Bir-MnO2 nanosheets were first obtained under the irradiation of a light source and acted as seeds for further decomposition of the excess MnO4- in the dark. The sizes of the Bir-MnO2 3D hierarchical nanostructures could be readily controlled by varying the concentration of KMnO4, the light source and the pH value. Furthermore, Bir-MnO2 nanosheets on CNTs were successfully fabricated utilizing the same strategy without the addition of any surfactants or organic templates. It is anticipated that the fabrication of Bir-MnO2 based nanostructures and composites could be beneficial from this universal method.