Single-atom site engineering has emerged as an effective strategy for enhancing the selectivity of multicarbon products in electrochemical CO₂ reduction reaction (CO₂RR), yet the synergistic roles of interfacial microenvironment regulation and key intermediate evolution remain poorly understood. Herein, Ag single-atom-modified porous CuO nanosheets (Ag₁-p-CuO) were constructed, and the mechanism by which Ag₁ atoms regulate the evolution of interfacial intermediates and water structure to promote ethylene formation, was systematically investigated through in situ shell-isolated nanoparticle enhanced Raman spectroscopy and advanced constant-potential density functional theory calculations. The atomically dispersed 0.2 % Ag species on porous CuO enabled a maximum C₂H₄ Faradaic efficiency of 54% and C₂₊ Faradaic efficiency of 78%. In situ Raman spectra reveal that, Ag₁ decoration enriches CO coverage at defective Cu step sites and reorganizes the interfacial water layer near the surface. The resulting Ag₁-Cu interface enhances cation-H₂O dissociation, establishing a local proton-donating environment that directs C–C bond formation toward a favorable asymmetric CO–CHO coupling pathway while lowering the hydrogenation barrier of the key ethylene precursor *CH₂CHO, thereby markedly improving ethylene selectivity. This work provides new insights into single-atom catalysis mediated interfacial microenvironment and intermediate behavior for promoting multicarbon product formation in CO₂RR.