The invention relates to a preparation method for nanometer manganese dioxide MnO2, which comprises the specific steps of: dissolving potassium permanganate and a reducer of methanamide or N, N-dimethyl formamide in de-ionized water to form uniformly mixed solution; reacting the mixed solution at the temperature of 20-100 DEG C for 1/6-96 hours, centrifugating and collecting product, washing and drying the obtained product to obtain monodispersive efflorescent and nemaline nanometer manganese dioxide; or reacting the mixed solution at 100-170 DEG C for 1/6-14 hours, centrifugating and collecting product, washing, drying and calcining the obtained product to obtain the bar-shaped nanometer manganese dioxide. The method can synthesize large quantity of monodispersive efflorescent, nemaline and bar-shaped nanometer manganese dioxide under low temperature by using simple devices and cheap and easily available raw materials. The amorphous monodispersive efflorescent nanometer manganese dioxide which has relatively large specific surface area has the advantages of enlarging the contact between the active substances and electrodes, reducing the internal resistance of cells, improving thediffusibility of ions and improving the utilization rate of the active substances.