Surface composition control plays a crucial role in electrocatalytic reactions for Pt-based bimetallic materials. However, strategies for controlling their surface composition always require extreme conditions including the use of harsh agents, the need for high temperatures, and the necessity of complex procedures. Here we develop a photo-assisted method to fabricate Au@PtAu core-shell nanoparticles (NPs) with well-tuned surface composition at ambient temperatures. It was found that their surface composition can be continuously regulated by the addition amount of HAuCl4 under photo-irradiation. By incorporating Au atoms, the synthesized Au@PtAu NPs show significantly improved properties for methanol electro-oxidation, owing to the reduced CO formation and the weakened binding strength of CO on Pt. The catalytic activity and durability vary with the relative active surface area of Pt or Au (PtSurf. or AuSurf.), and the optimal PtSurf. was found to be ∼64% (AuSurf. ∼36%). The present work highlights the “photo-assisted method” as a green and effective method to synthesize advanced materials with optimized functional properties.