A combination of molecular dynamics (MD) and density functional theory (DFT) calculations were used to study the hydration structures of K+ and Na+ ions under the confinement of 18-crown-6 in order to identify the role of water in the selectivity of 18-crown-6 towards Kt The radial distribution functions, coordination numbers, orientation structures and interaction energies were analyzed to investigate the hydration of K+ and Na+ in 18-crown-6/cation complexes. All calculations of K+ and Na+ in bulk water were also conducted for comparison. The simulation results show that the orientation distributions of the water molecules in the first coordination shell of K+ are more sensitive to the confinement of 18-crown-6 than those of Na+ It is more favorable to confine a K+ in 18-crown-6 than a Na+ in terms of interaction energy. Good agreement is obtained between MD results and DFT results.