The invention relates to a graphene modified carbon fiber emulsion sizing agent and a preparation method thereof. The conventional solvent sizing agent is not environment-friendly. The sizing agent is a mixed system consisting of organic resin, an emulsifying agent, a lubricating agent, graphene, an organic solvent and deionized water, wherein the emulsifying agent, the lubricating agent, the graphene and the organic solvent are respectively 2 to 14 percent, 0.5 to 2 percent, 0.05 to 1 percent and 30 to 200 percent based on the mass of the organic resin; and the solid content of the mixed system is 0.1 to 5 percent. The preparation method of the sizing agent comprises the following steps of: dissolving the organic resin in the organic solvent to form organic resin solution, adding the emulsifying agent, the lubricating agent and the deionized water into the organic resin solution, and stirring until the mixed system is subjected to phase transfer; adding the deionized water until the solid content is 30 to 50 percent to obtain organic resin emulsion; and adding the graphene into the organic resin emulsion, and adding the deionized water to dilute the mixed system until the solid content is 0.1 to 5 percent so as to obtain the graphene modified sizing agent. The method is simple and reliable, and the prepared graphene modified sizing agent can well protect carbon fibers and effectively improve the interface property of the carbon fibers and matrix resin.