Electroreduction of CO2 (CO2RR) into high value‐added chemicals is an attractive route to achieve carbon neutrality. However, the development of an efficient catalyst for CO2RR is still largely by trial‐and‐error and is very time‐consuming. Herein, we built an electrocatalyst testing platform featuring a home‐built automatic flow cell to accelerate the discovery of efficient catalysts. A fast screening of 109 Cu‐based bimetallic catalysts in only 55 h identifies Mg combined with Cu as the best electrocatalyst for CO2 to C2+ products. The thus designed Mg−Cu catalyst achieves a Faradaic efficiency (FE) of C2+ products up to 80 % with a current density of 1.0 A cm−2 at −0.77 V versus reversible hydrogen electrode (RHE). Systematic experiments with in situ spectroelectrochemistry analyses show that Mg2+ species stabilize Cu+ sites during CO2RR and promote the CO2 activation, thus enhancing the *CO coverage to promote C−C coupling.