The ever-increasing power density and high integration of electronic devices and components greatly promotes the demand for thermal management materials. Herein, we report a lightweight and thermal conductive polymer composite based on the epoxy resin (EP) and high graphitized pitch-based carbon fiber (CF). To address the problem of the surface inertness of the raw CF (RCF), a Dies-Alder reaction was introduced after a conventional oxidation by AgNO₃/K₂S₂O₈. The results of the X-ray photoelectron spectroscopy, Raman spectroscopy and scanning electron microscopy showed that the oxygen content of the modified CF (OCF-MA) surface was significantly increased but its structure was not damaged after the Dies-Alder reaction with the maleic anhydride. Because of the superior thermal conductivity and improved surface activity of the RCF-MA, the OCF-MA/EP composites showed a remarkably high thermal conductivity of 4.43 W/m·K at a relatively low CF loading of 11.5 vol%, which is 2.34 times that of the RCF/EP composite (1.89 W/m·K). The infrared thermal imaging method further demonstrated the excellent heat transfer advantage of the RCF-MA/EP composites over the neat EP and RCF/EP composites. The thermogravimetric analysis showed that the addition of OCF-MA has no adverse effect on the thermal stability of the EP composites. In addition, the tensile test showed that the addition of OCF-MA into the EP had a positive effect on the tensile strength of the EP. This work demonstrates that grafting maleic anhydride via Diels-Alder reaction could be a feasible method to functionalize CF for advanced thermal management applications.