In carbon fiber reinforced thermoplastic composites (CFRTP), the strength of the interface between the carbon fiber (CF) and the thermoplastic resin matrix has a significant impact on its overall mechanical properties. To improve the interfacial properties between CF and polyamide 6 (PA6), nano-sized Fe₃O₄ was introduced to the CF surface for synergistic modification using the polymerization and adhesion characteristics of dopamine (DA). Through simple immersion co-deposition, the poly-dopamine/nano-Fe₃O₄ (PDA/Fe₃O₄) coating was constructed on the CF surface by oxidative self-polymerization of DA. The microstructure and chemical characteristics of the CF surface before and after modification were characterized. The results show that adding the PDA/Fe₃O₄ sizing agent significantly improves the roughness and chemical activity of the CF surface, and strengthens the mechanical engagement and chemical interaction between the fiber and resin matrix. Compared with unmodified CF/PA6 composites, the prepared modified CF/PA6 composites exhibit excellent mechanical properties, with their bending strength and interlaminar shear strength increased by 30.8% and 29.1%, respectively.