The invention relates to amorphous MnO2 and preparation method and application thereof. The preparation method comprises the following steps of (1) mixing a KMnO4 solution and a Mn(CH<3>COO)<2>.4H<2>Oto prepare a uniform solution; (2) adding a KOH solution into the uniform solution obtained in the step (1), and performing stirring to obtain a deep brown precipitant; and (3) washing, drying, grinding and calcining the deep brown precipitant obtained in the step (2) to obtain an amorphous MnO2 material. The prepared amorphous MnO2 is used for a sodium ion battery positive electrode material. Compared with the prior art, the synthesis process is simple, the reaction condition is simple, the reaction environment requirement is low, and the preparation method is suitable of industrialization on a large scale; the amorphous MnO2 material is used as the sodium ion battery positive electrode material and has relatively large specific area, the sodium ion mobility is greatly improved, relatively high specific capacity and favorable cycle stability are shown.