The invention discloses composite nanometer carbon paper and a preparation method thereof. The nanometer carbon paper uses carbon nanometer paper as a framework network, MnO2 metallic oxides are deposited on the surface, the outer layer is covered with grapheme, and the composite nanometer carbon paper belongs to a flexible composite film material. The preparation method of the composite nanometer carbon paper comprises the steps that carbon nanometer tubes are dispersed in a solvent, then, the suction filtering is carried out to obtain carbon nanometer tube paper, next, the carbon nanometer tube paper is used as a carrier for depositing the MnO2 metallic oxides on the surface, and finally, the grapheme is adhered on the product surface to obtain target products. The electrical conductivity, the heavy current charging and discharging capability, the specific capacitance and the cycle life of the composite nanometer carbon paper are respectively and obviously enhanced through being compared with those of the carbon nanometer tube-MnO2, the problems that in the existing super capacitor, the metallic oxides easily fall off from the surface of the carbon nanometer tubes, the electrical conductivity of the metallic oxides is poor, and the like are solved, the composite nanometer carbon paper also has the characteristics of light weight and flexibility of materials and high efficiency, simultaneously, the preparation process is simple, the operation is easy, the controllability is good, and in addition, the cost is low. The composite nanometer carbon paper and the preparation method have wide application prospects in fields of new energy sources, advanced chemical engineering, electronic devices, film preparation and the like.