In this paper, a simple and sensitive immunosensor was fabricated for the detection of alpha-fetoprotein (AFP), which is based on a one-pot synthesized nanocomposite of gold nanoparticles-dextran-reduced graphene oxide (AuNPs-Dex-RGO). The AuNPs-Dex-RGO nanocomposite structure was confirmed by transmission electron microscope (TEM), Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR). With ferricyanide as the electrochemical probe, this method achieved ultrasensitive detection of AFP antigen. The specific immunoreaction between AFP and AFP antibody resulted in a linear relationship between the redox peak current intensity of [Fe(CN)(6)](4-/3-) and AFP concentration. The detection range is 0.01-20 ng mL(-1), and the detection limit is 0.05 pg mL(-1). Moreover, the selectivity, reproducibility and stability of the proposed immunosensor were acceptable, which permitted the feasibility for the determination of AFP in human serum with satisfactory results, indicating the great potential for clinical application.