A series of Au@Pt composite nanoparticles was synthesized by photochemical coreduction in the PEG-acetone aqueous solutions containing Au(III) and Pt(IV) ions with different molar ratios. The resulting composites were supported on carbon black for preparing Au@Pt/C catalysts, respectively. By the characterization of UV-Vis spectrum, TEM and HR-TEM, a spherical Au core Pt shell composite nano-structure was confirmed. The chemical state and structure character of the composite nanoparticles and the catalytic property of Au@Pt/C catalysts were analyzed by XPS, EDS and electrochemical method, respectively. The results show that all of Au@Pt/C catalysts with different Au:Pt molar ratios have high catalytic activity and stability for methanol oxidation; among them, the catalytic activity of the Au@Pt/C catalyst formed at Au:Pt=1:1 is the highest and is about 4 times than that of commercial Pt/C catalyst. The primary cause of high catalytic activity of the core/shell structure was discussed.