Carbon fiber was surface modified with dopamine via self-polymerizing deposition process, then the polyamide 6-based composites (CF/PA6) were prepared with the modified carbon fiber as reinforcement. The morphology, roughness, wettability and chemical construction of carbon fiber were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), contact angle measuring device, Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS). The effect of deposition time of polydopamine (PDA) on the interface mechanical properties of the composites was also investigated. The results show that the surface of the modified carbon fiber was covered by a uniform PDA film, which significantly increased the surface activity, surface roughness and chemical bond energy of the carbon fiber, and greatly improved the interfacial compatibility between the carbon fiber and polyamide 6 matrix. The optimal PDA deposition time for the modification of carbon fiber was 16h, correspondingly, with the optimally modified carbon fiber as reinforcement, the interlaminar shear strength and bending strength of composites reached 31.7 MPa and 308.2 MPa, which was 72.3% and 56.9% higher respectively than those of the ones reinforced with the blank carbon fiber.