Modulation of bimetallic nanocatalysts with atomic precision would allow for significant increases in catalyst activity through the optimization of heteroatomic interplay. In practice, this level of control over homogeneously turning the heteroatomic states in the surface regions of catalysts remains a great challenge for practical applications requiring mass production. In this work, a one-step aqueous strategy for preparation of AuPd nanoicosahedra core-shell structures has been developed. By turning the molar ratio of Au and Pd precursors, the nanoicosahedra could be made with uniform core composition (Pd ∼20%) and largely flexible surface compositions (Pd: 20-70%). Under model reactions of electrochemical ethanol oxidation (EOR) and 4-nitrophenol (4-NP) reduction, the Au55Pd45 surface displayed superior activities. X-ray absorption spectroscopy revealed that AuPd nanoicosahedra with Pd-poor surfaces (Au79Pd21 and Au60Pd40) maintain Au-like electronic structures while those with Pd-rich surfaces are more Pd-like. The Au55Pd45 surface struck a delicate balance between the two extremes, leading to the optimal Au-Pd interplay for catalytic performance. Overall, the development of a one-step aqueous strategy for atomic-level control over bimetallic nanoparticle surface states is an important milestone for future advances in catalysis. Our findings in atomic-scale catalyst design will bring deep insights into the fields of nanosynthesis and heterogeneous catalysis, which will be beneficial for further scientific breakthroughs.