Various 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4)/water mixture with varying concentrations were studied by molecular dynamics simulations. The radial distribution function, the power spectrum velocity-velocity correlation function were calculated to reveal the microstructure and the IR spectrum. In both liquids, with the concentration of water in the mixture increase, the rotation bands and the bending bands of water are red shift whereas the O-H stretch bands are blue shift. It was found that the molecular rotation motions become slower as the proportion of water increases and water molecules tend to be isolated from each other in mixtures with more ions than water molecules. It was shown that water molecules tend to be isolated from each other in mixtures with more ions than water molecules in both liquids.