Molecular dynamics was utilized to investigate the properties of ChCl/urea (1:2)/water mixtures from pure ChCl/urea to infinitely dilute solution. To further study the mechanism at molecular scale, the local microstructure variation in the mixtures with the increase of water content was analyzed in detail. Simulation results showed that neither choline cations nor chloride anions are saturated by the coordinated water molecules, even when the water molar fraction reaches 0.9. The hydration number proportions for different ions indicated that the hydration strength of chloride anion is stronger than that of choline cation, which may play greater effects on the properties of mixtures. This result is further confirmed by the analysis of interaction energy between cation (anion) and water molecules. Moreover, the ion pairing between choline cations and chloride anions is a dominant factor to affect the solution properties at lower water content, whereas the hydration turns to be the dominant factor with increasing water content. The competition between ion pairing and ionic hydration could be the intrinsic mechanism resulting in non-ideal properties.