Electrochemical CO2 reduction to multicarbon (C2+) products offers a promising route toward carbon neutrality. However, operational robustness remains a major challenge, as Cu catalysts generally achieve high C2+ Faradaic efficiency (FE) only within narrow operating windows, limiting practical deployment under dynamic conditions. Here, guided by dual-descriptor screening, we identify alkaline earth metals as a distinctive class of Cu modifiers that balance *CO adsorption with *CO-to-*CHO hydrogenation, thereby favoring asymmetric C–C coupling and enabling robust C2+ electrosynthesis. Among them, Sr-Cu is identified as the most effective catalyst and reconstructs into a stable Sr2Cu(OH)6/Cu interface under reaction conditions. In a flow cell, it maintains C2+ FE above 80% over 0.4–1.2 A cm–2, reaching 91% at 1.0 A cm–2, and retains high C2+ selectivity across broad pH, CO2 flow rates and K+ concentration windows, with a maximum C2+ single-pass carbon efficiency of 62%. This robustness extends to a full cell, where Sr-Cu delivers 92% C2+ FE and 35% C2+ energy conversion efficiency. These findings establish alkaline-earth/copper catalysts as a robust platform for C2+ electrosynthesis under dynamic operating conditions.