Unveiling the so far ambiguous mechanism of the significant dependence on the identity of alkali metal cation would prompt opportunities to solve the more than two orders of magnitude slowdown of hydrogen electrocatalytic kinetics in base relative to acid, which has hampered the effort to reduce the precious metal usage in fuel cells by using the hydroxide exchange membrane. Herein, we present atomic-scale evidences from ab-initio molecular dynamics simulation and in-situ surface-enhanced infrared absorption spectroscopy which show that it is the apparent discrepancies in the electric double-layer structures induced by differently sized cations that lead to largely different interfacial proton transfer barriers and therefore hydrogen electrocatalytic kinetics in base. Concretely, severe accumulation of larger cation in electric double-layer causes more discontinuous interfacial water distribution and H-bond network, thus rendering the proton transfer from bulk to interface more obstructed. Such notion is strikingly different from the previously envisioned impact of cation-intermediate interactions on the energetics of surface steps, providing a unique interfacial perspective for understanding the ubiquitous cation specificity in electrocatalysis.