Electrochemical hydrogen evolution reaction (HER) on silver is significantly accelerated by adding 4,4'-bipyridine (BiPy) into neutral or alkaline aqueous solution (T. Uchida et al. J. Am. Chem. Soc., 130 (2008) 10826). The mechanism of the accelerated HER by BiPy is studied by the first-principles molecular dynamics simulation with the constant Fermi energy scheme of the effective screening medium (ESM) method. The simulation shows that H-2 is evolved through the following three steps: First, BiPy adsorbed on the electrode via one N-end directing another N-end toward the solution is reduced to N-hydro BiPy radical (BiPy-H center dot), in which H is bonded to N atom of the solution side. BiPy-H center dot is further reduced to N,C(3)-dihydro BiPy with an activation free energy of 0.5 eV by the second proton-coupled electron transfer. Finally, H-2 is released from the two H atoms on N and C(3) of N,C(3)-dihydro BiPy by the assist of H2O or OH-. BiPy is reproduced by the H-2 evolution and the processes are catalytic-cycled. (C) 2017 Elsevier B.V. All rights reserved.