The invention relates to a preparation method and application of a mesoporous manganese oxide/carbon composite nanometer material. The preparation method comprises the following steps of taking a nonionic surfactant as a template agent, an organic silicon source and a high-molecular polymer as an organic precursor and an inorganic manganese source as an inorganic precursor, synthesizing a manganese oxide/carbon/silicon dioxide composite material by an evaporation-induced self-assembly method, and further removing silicon dioxide, thereby obtaining the mesoporous manganese oxide/carbon composite nanometer material with relatively large specific area and pore volume and large aperture, wherein the specific area is 297-487m<2>/g, the pore volume is 0.19-0.34cm<2>/g, the aperture is 2.0-3.4 nanometers, and the mesoporous manganese oxide/carbon composite nanometer material can be used for fabricating an electrode material used for a supercapacitor. By the preparation method, the mesoporous manganese oxide/carbon composite nanometer material with relatively large specific area and pore volume and large aperture is obtained; and moreover, the preparation method is simple, is low in production and is suitable for mass production.