Proton transfer (PT) at the catalyst-electrolyte interface is a fundamental step in the hydrogen evolution reaction (HER), yet its interfacial dynamics is poorly understood. Here, we report 15% Co/O-FeP supported on multiwalled carbon nanotubes (MWCNTs) as a highly efficient HER catalyst, delivering an overpotential of only 58 mV at 10 mA·cm⁻² and sustaining 500 mA·cm⁻² for over 111 h without noticeable degradation. Outstanding HER performance and high-current stability make industrial-level hydrogen evolution feasible. Using in-situ Raman spectroscopy and ab initio molecular dynamics (AIMD) simulations, we reveal that the orientation of interfacial water molecules, particularly interfacial H⁺ solvation effect induced by H-down configurations, governs HER kinetics by accelerating proton migration and enabling a concerted proton-electron transfer (CPET). Mechanically, PT proceeds through a two-step pathway: (i) Protons migrate from bulk solution to the electrical double layer (EDL) to form H₃O⁺ and (ii) H₃O⁺ undergoes rapid proton exchange with catalyst active sites. H-down water configurations within the inner Helmholtz plane (IHP) stabilizes de-solvation shells and couples efficiently with interfacial electrons, lowering kinetic barriers. These findings establish a direct correlation between interfacial water structure, hydrogen-bond network, and CPET efficiency, providing fundamental insights for the rational design of advanced electrocatalytic interfaces.