Abstract: The interfacial interactions between carbon fiber (CF) and polyetherketoneketone (PEKK) are poor due to the chemical inertness of CF, limiting the mechanical properties of carbon fiber reinforced polyetherketoneketone (CF/PEKK) composites. In this study, a water-based sizing agent of sulfonated polyetherketoneketone (SPEKK) was prepared by sulfonating domestic PEKK resin, in order to improve the mechanical properties of CF/PEKK composites. By regulating the sulfonation degree, the stable SPEKK aqueous emulsion was obtained and used for CF sizing modification. Subsequently, CF/PEKK composites were fabricated via vacuum hot-press technique. When the concentration of SPEKK aqueous emulsion was 0.5wt%, the flexural strength, flexural modulus and interlaminar shear strength of the modified CF/PEKK composites reached 1237 MPa, 78 GPa, and 92 MPa, which were 35.5%, 5.4%, and 26.0% higher than that of the unmodified CF/PEKK composites, respectively. Such enhancement in mechanical properties could be attributed to the introduced SPEKK on the CF surfaces, which could form hydrogen bonds and π-π interactions with CF, as well as π-π interactions and diffusion and entanglement with PEKK, significantly promoting the interfacial bonding between CF and PEKK. As a simpler process, CF surface modification with water-based SPEKK sizing agent is environmentally friendly and suitable for the industrial production, which is of great significance for the development of domestic high-performance carbon fiber reinforced thermoplastic composites.