The invention discloses a method for preparing nano manganese dioxide of different crystal forms and appearances by adopting microwave hydrothermal method. The steps are as flows: (1) dissolving potassium hypermanganate in water, adding hydrochloric acid, and stirring to prepare a reaction solution; (2) pouring the reaction solution into a sealable reactor, placing the reactor in a microwave field for microwave radiation with microwave frequency being 2.45GHz, temperature being 105-115 DEG C, and heat preservation time being 0.5-2h, separating, washing and drying the generated precursor to obtain petal-shaped nano delta-phase manganese dioxide. In step (2), the temperature of the reaction solution is 135-145 DEG C, and the product of the reaction is alpha-phase manganese dioxide nanotube. The invention features simple technique, low cost, controllable product quality, extensive applicability and the like, the prepared nano manganese dioxide is applicable to fundamental theoretical research in such fields as battery, molecular sieve, catalyst and magnetic materials, and the research and development of related function materials.