The invention discloses a method using acrylamide aqueous solution grafting to modify a carbon fiber surface sizing agent, relates to methods for modifying the carbon fiber surface sizing agent, and aims to solve the technical problem that the interface bonding strength of current carbon fiber reinforced vinyl ester resin or unsaturated polyester resin composite materials is low. The method includes the steps of firstly, preparing an acrylamide modifier aqueous solution; secondly performing carbon fiber surface modification. The method has the advantages that the method is simple and practicable, chemical grafting is directly performed on the carbon fiber surface sizing agent, the strength of carbon fibers is not damaged, and the modified sizing agent can protect the carbon fibers; the interface shear strength of the modified carbon fiber/unsaturated resin is increased evidently, and the mechanical performance of the composite materials of the modified carbon fiber/unsaturated resin is increased evidently.