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