Possible adsorption structures of H2O on (TiO2)n(n=3—6) clusters were studied by using genetic algorithm combined with empirical potential function. The structures were further optimized using DFT method B3LYP/6-31G**. The results show that H2O molecule is adsorbed on TiO2 clusters through the attachment of oxygen atom to the titanium atom with lower coordination number. Molecular orbitals show that the bond between water and cluster is formed mainly by the 3s3p orbitals of titanium，the orbitals of H2O keep the basic features unchanged but are more delocalized after adsorption. Combined with population analysis and calculation of vibration frequencies，the results show that the O—H bond becomes weaker after H2O is adsorbed on TiO2 clusters.