The direct electrochemical conversion of CO₂ to syngas with controllable composition remains challenging. In this work, driven by concentration gradient, a simple air-heating aided strategy has been developed to adjust surface composition of the self-supporting nanoporous AuCu₃ alloy. According to Fick First Law, the interior Cu atoms of the AuCu₃ alloy with Au-rich surface gradually segregated outwards during heating, realizing Cu-rich surface eventually. Correspondingly, the competing electrocatalytic CO₂ reduction (ECR) to CO and hydrogen evolution reactions (HER) were tactfully balanced on these alloy surfaces, thus achieving proportion-tunable syngas (CO/H₂). Density functional theory (DFT) calculations on the Gibbs free energy change of the COOH* and H* (ΔG_COOH*, ΔG_H*) on the alloy surfaces were conducted, which are generally considered as the selectivity descriptors for CO and H₂ products, respectively. It shows ΔG_COOH* gradually increases in contrast to the decreased ΔG_H* with more Cu on the surface, suggesting H₂ is more favored over Cu sites, which is consistent with the declining CO/H₂ ratio observed in the experiments. This study reveals that the surface composition controls ECR activity of nanoporous AuCu₃ alloy, providing an alternative way to the syngas production with desirable proportion.