The work aims to prepare a thermoplastic sizing agent to improve the surface properties and monofilament tensile properties of CF.The sulfonated poly (ether ether ketone) (SPEEK) sizing agent was prepared by a simple sulfonation process, and was applied to size the unsized T300 carbon fiber (CF) by spraying. The structural performance and properties of the sizing agent and its effect on the properties of CF were studied. The results demonstrated that the degree of sulfonation of SPEEK was about 74%. The water borne sizing agent prepared with it as the main slurry had good film-forming property and storage stability. After sizing treatment, a continuous SPEEK sizing layer was formed on the surface of CF. Also, the surface functional groups of SPEEK sized CF increased significantly, and the contact angle of resin droplets on the surface of fiber decreased. Besides, the sizing layer can heal some defects of the CF surface. Thus, compared with unsized CF, the tensile strength of SPEEK sized CF was increased by 5%. Therefore, the SPEEK sizing treatment can improve the surface properties and monofilament tensile properties of CF.