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 K+.The radial distribution functions,coordination num-bers,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. 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 K +. The radial distribution functions, coordination num-bers, 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 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.