Generally speaking, the preparation of single-atom catalysts always requires harsh conditions such as high-temperature pyrolysis or strong acid etching. In this manuscript, a simple and effective plasma-activated strategy is employed to synthesize a MOF-based single-atom copper catalyst. The bombardment of plasma forms abundant oxygen vacancies and significantly increases the number of low-coordinated catalytically active copper sites. Moreover, plasma treatment also creates a hierarchically porous structure, which can effectively adsorb the reactant molecules. The synergistic effect of the porous structure and low-coordinated copper sites dramatically improves the activity of CO₂ electroreduction to CH₄ with a maximum faradaic efficiency of 75.3%. Furthermore, the total faradaic efficiency of carbon-containing products (CO, CH₄ and C₂H₄) can reach as high as 96.5% with a partial current density of 47.8 mA cm⁻². Density functional theory (DFT) calculations confirm that the low-coordinated copper sites can be beneficial for the formation and further reduction of the key intermediate to CH₄. This strategy provides a successful example for the preparation of single-atom catalysts under mild conditions.