To promote application of graphene fiber as electrode for supercapacitors, carbon nanotubes (CNTs)/reduced graphene oxide (RGO) composite fibers were fabricated with GO/polyacrylonitrile (PAN) fibers using chemical vapor deposition method, and then treated with mixed acid to modify the hydrophilicity of the fiber. Transformation of excellent nano-scale properties of graphene and CNTs into macroscopic material properties was realized. The GO/PAN fiber was prepared by wet spinning and thermal stabilized in advance before in situ growth of CNTs to the fiber. Structures and electrochemical properties of the CNTs/RGO composite fiber were investigated. Interaction between GO and PAN during stabilization resulted in less mass loss of the fiber. The CNTs/RGO composite fibers exhibited a specific surface area of 87.873 m² g⁻¹ with large quantity of entwined CNTs about 39–130 nm in diameter and centimeters in length. Increase of 37.43% oxygen-containing functional groups greatly improved the hydrophilicity of the fiber after chemical modification. The specific capacitance increased from 34 to 141 F g⁻¹ at 0.1 A g⁻¹ for the CNTs/RGO composite fibers due to improved ionic accessibility and pseudo-capacitance provided by oxygen-containing functional groups. The chemical modified CNTs/RGO fiber is suitable for electrode material of supercapacitors due to its excellent cycle stability with specific capacitance retention of 96% after 2500 cycles of the charge/discharge process and low ESR and R_ct. A method for the synthesis of CNTs/RGO composite fibers with good three-dimensional structures and an idea of modifying ionic accessibility to improve the electrochemical performance of the fibers was presented.