AbstractThe effects of sulfonated polyether sulfone (s‐PSF) with different sulfonation degrees on the interfacial properties of carbon fiber (CF)/poly(ether‐ether‐ketone) (PEEK) composites were investigated systematically. The performance of the modified CF and the corresponding CF/PEEK composites and was tested and characterized. Test results show that the CF surface polarity increases, the surface contact angle decreases, and the surface free energy increases with the increase in the s‐PSF sulfonation degree. Scanning electron microscopy analysis shows that the increase in the sulfonation degree of s‐PSF is beneficial to improve the interface between CF and PEEK. This condition can be ascribed to the hydrogen bonding force between the s‐PSF sulfonic acid group and the polar functional group on the surface of the modified CF and the compatibility between s‐PSF and PEEK. In terms of physical properties, the thermal and mechanical properties of CF/PEEK composite are improved with the increase in s‐PSF sulfonation degree. The interlaminar shear strength, flexural strength, and modulus of CF/PEEK composites increase by 60.16%, 30.27%, and 19.30%, respectively.