We report for the first time a novel and rapid (1 h) synthesis method for birnessite-type MnO2 nanostructures via a polyol-reflux process through oxidizing MnCl2 with H2O2 under basic conditions in the presence of polyvinylpyrrolidone (PVP). Influencing factors such as the dosage of reactants and the reaction times are systematically investigated. The molar ratios of OH/Mn played an important role in the formation of birnessite-type MnO2 nanostructure with good crystallinity and ordered 3D nanostructures. A possible formation mechanism for the nanostructure was proposed. The flower-like birnessite-type MnO2 nanostructure is composed of nanosheets with an average diameter ca. 300-500 nm and shows mesoporous characteristics with a pore diameter of 20 nm. Due to its unique mesoporous structure, the birnessite-type MnO2 exhibits excellent ability to remove organic pollutants (Ponceau 2R) and shows a potential application as an electrochemical capacitor.