An application of mixed quantum-classical molecular dynamics to vibrational relaxation of the solute molecule in the solution has been investigated. In the present paper, we demonstrate that mean field approximation, rather than surface hopping approximation, works satisfactorily well for the vibrational relaxation of, at least, the CN- ion in the aqueous solution, where the potential surface of the solvent water depends little on the vibrational quantum state of the solute. The calculated relaxation time is in good correspondence to those obtained from other two different methods based upon the same potential model, i.e., Fermi's Golden Rule with classical force autocorrelation function and path integral influence functional theory in its classical bath limit. The present method gives some interesting findings for the relaxation. For example, the energy relaxation time may be longer than the population relaxation time. With respect to the coupling to the solvent, water molecules in the first hydration shell make a leading contribution to the relaxation of the solute. (C) 2001 American Institute of Physics.