To enhance the mechanical properties of carbon-fiber-reinforced polymer composites (CFRPs), this study devised a novel three-dimensional web-like “rigid–flexible” surface modification strategy. The synergistic incorporation of carbon nanotubes (CNTs), polydopamine (PDA), and cellulose nanofibers (CNFs) constructed a “three-dimensional nanospider web” modulus transition layer. The modified carbon-fiber (CF-0.1%CNT-PDA-CNF) surface exhibits a three-dimensional network structure, with significantly increased surface roughness. The surface energy increased by 128.60% compared to the desized carbon fiber, thereby improving the wettability of the carbon-fiber surface. The results of both PeakForce-Quantitative Nanomechanical Mapping (PF-QNM) and EDS analyses indicate that a transition layer of a certain thickness initially formed at the interface. At the interface, the modulus exhibits a gradual gradient decrease from carbon fiber to epoxy resin, achieving more efficient stress transfer. The interfacial shear strength (IFSS, 95.71 MPa), interlaminar shear strength (ILSS, 73.19 MPa), tensile strength (701.08 MPa), and flexural strength (934.41 MPa) of the CF-0.1%CNT-PDA-CNF/EP composite material increased by 38.39%, 54.93%, 51.74%, and 64.98%, respectively, compared to the composite material made from desized carbon fiber. Through hydrogen bonding, covalent bonding, and π-π interactions, CNTs, CNFs and PDA formed a “rigid–flexible” transition layer with a modulus gradient at the CF-epoxy interface, achieving a significant enhancement in the mechanical properties of the composite material.