Vibrational energy relaxation of azide anion in water was investigated with molecular dynamics simulation. The Landau-Teller formula without the solute electronic polarization exceedingly underestimated the relaxation rate of the antisymmetric stretching mode, and thus various relaxation mechanisms were comprehensively evaluated to elucidate the fast relaxation. As a result, the direct relaxation to the vibrational ground state and the intramolecular vibrational redistribution (IVR) to the symmetric stretching mode have equally dominant contributions to the decay rate. Intramolecular charge fluctuation induced by the solute vibrational motion greatly enhanced the friction by the solvent to reproduce the experimental rate fairly well. The isotope effect of H2O and D2O was also elucidated along the present mechanism.