As an important transition metal oxide, manganese dioxide (MnO₂) has attracted more and more attention due to its abundant reserves, varied crystal types and excellent material properties. Nanostructured MnO₂ has smaller size and larger specific surface area, that makes it can further optimize its material properties and expand its application fields. In the introduction, this article starts with the introduction of the application of manganese dioxide, and points out that nanostructuring and variability in crystal form have an important influence on the structure and properties of manganese dioxide. The main text summarizes and reviews the research progress in recent years from two aspects: the preparation methods and the applications of nanostructured MnO₂. (1) This paper introduces the progress in the preparation methods of nanostructured MnO₂ including hydrothermal, Sol-Gel, chemical precipitation, solid-phase synthesis. Then the advantages and disadvantages of preparation methods, the morphologies and properties of nanostructured MnO₂ are summarized. (2) The applications of nanostructured MnO₂ including energy-storage electrodes, electrochromic devices, catalysts and bio-sensors are reviewed. Nanostructured MnO₂ can be used as the cathode material of batteries and the electrode material of supercapacitors. Manganese- containing composite oxides prepared by adjusting the crystal form of MnO₂ and compounding are used as the cathode material of the lithium ion batteries, which can increase the capacities and improve the cycle stability of batteries. It has been industrialized as a cathode material for lithium-ion power batteries, and has good application prospects in the field of new energy vehicles. As the electrode material of electrochromic devices, MnO₂ is usually used by combining with other materials with large optical modulations since the color of pure MnO₂ mainly changes between brown and yellow and its optical modulation is small. For example, polyaniline/MnO₂ hybrid electrochromic film has a great difference in morphology, structure and electrochromic performance compared with pure polyaniline film, showing higher optical modulation, coloration efficiency and cycle stability. Nanostructured MnO₂ plays important roles in the catalytic conversion of ethylbenzene and the catalytic elimination of air pollutants. Nanostructured MnO₂ can increase the current response, reduce the detection limit, and greatly improve the sensitivity of detection. In recent years, it has been gradually paid attention to and widely used in the field of biosensors. For example, MnO₂ nanosheets assisted fluorescence polarization biosensors can be effective in detection of Ag⁺ in environmental water samples, PtAu-MnO₂ binary nanostructures modified graphene paper show good sensing performance in non-enzymatic glucose detection. In conclusion part, current existing problems are analyzed. The development direction of nanostructured MnO₂ applied in lithium-ion battery cathode materials and electrochromic devices are pointed out. The future prospects for development of nanostructured MnO₂ are discussed.