The invention relates to a method for preparing a functionalized carbon material, and belongs to the technical field of novel materials. According to the method, potassium ferrate is adopted to oxidize a carbon material under the action of mechanical grinding force, and thus the functionalized carbon material containing oxygen-containing functional groups is prepared. Compared with the prior art, a preparation method of ''green oxidant+green reaction processes'' is adopted. On one hand, the used potassium ferrate is green and harmless oxidant, on the other hand, no solvent is used in a reaction, and the carbon material and the oxidant are refined and mixed under the action of the mechanical force, and are reacted through collision and friction. As the green oxidant is used, and no solvent is needed, the method is an environmental-friendly functionalized carbon material preparation method, and has large popularization values.