The electrochemical reduction of CO2 to CH4 offers a promising pathway for closing the carbon cycle and enabling sustainable energy conversion, yet it is fundamentally limited by sluggish reaction kinetics and competing side reactions. We report the controlled fabrication of core-shell Cu@Zn nanocluster catalysts (Cu@Zn NCs) via cluster beam deposition (CBD). Molecular dynamics simulations based on the NEP89 universal neuro- evolution potential reveal a spontaneous formation mechanism of the core-shell architecture. Aberration- corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) further confirms uniform quasi-spherical Cu@Zn NCs with an average size of ~5.86 nm, a well-crystallized Cu (111) core, and a continuous Zn shell. In electrocatalytic CO2 reduction, the Cu@Zn NCs achieve a CH4 Faradaic efficiency of up to 63.1% with a partial current density of 126.2 mA cm2 at 1.5 V vs. RHE and a total current density of 200 mA cm2. Mechanistic investigations combining in situ infrared spectroscopy and hydrogen/ deuterium (H/D) kinetic isotope effects (KIE) analysis reveal that the Zn shell accelerates interfacial water dissociation, providing an abundant local proton source, while simultaneously strengthening *CO adsorption on Cu sites and promoting its hydrogenation to the key *CHO intermediate. This synergistic effect suppresses *CO desorption and C–C coupling pathways, thereby steering the reaction toward highly selective CH4 formation. This work highlights the effectiveness of precise interfacial engineering in bimetallic core-shell catalysts and provides mechanistic insights for the rational design of CH4 selective electrocatalysts for deep CO2 hydrogenation.