In summary, by using alkali cations to suppress hydronium reduction and promote CO2 reduction, we demonstrated efficient CO2 electroreduction in strongly acidic medium. We showed that this approach is universal for various catalysts and cations, and we revealed cationinduced modulation of electric field as the origin of the cation effects. This work provides a promising strategy to avoid the carbonate problem in CO2 electroreduction, which is one of the main road blockers for low-temperature CO2 electrolysis.