The invention discloses a method for coating surfaces of carbon fibers with polyethyleneimine in supercritical methanol, relates to a method for carrying out surface modification on the carbon fibers in the supercritical methanol and aims to solve the problems of damaging a body structure of the carbon fibers and reducing intensity of the carbon fibers of the existing carbon fiber surface modification method. The method comprises the following steps: 1, carrying out extraction processing on the carbon fibers; and 2, coating to obtain the carbon fibers with the surfaces coated with the polyethyleneimine. The method has the advantages that 1, supercritical fluid adopted in the method has higher permeation and mass transfer properties, so that the surfaces of the carbon fibers, which are coated with the polyethyleneimine, are uniform and compact; 2, the body structure of the carbon fibers is not damaged and the tensile property cannot be lost; 3, with adoption of the method, less time is consumed, the operation is simple and the implementation is easy; and 4, with adoption of the method, alcohol is used as a supercritical medium, so that the environment pollution is avoided and environmental protection is facilitated. According to the invention, the method for coating the surfaces of the carbon fibers with the polyethyleneimine in the supercritical methanol can be obtained.