Abstract: A periodical interaction model of LDHs-Cl-yH2O has been proposed. The geometry optimization and energy of the layered double hydroxides (LDHs) were calculated using CASTEP/LDAmethod at the CA-PZ level. The distribution of H2O in the interlayer and the super-molecular interaction between host layer and guest anion have been investigated by analyzing the geometric parameters, charge population, energy, and density of state (DOS ). The results showed that there was a strong super-molecular interaction between the host layer and the guest anion Cl -. In the system of LDHs-Cl-yH2O, the interlayer distance increased gradually then tended to invariableness. And in the process of hydration of LDHs-Cl, hydrogen bonding was superior to electrostatic interaction, and layer-water type hydrogen bonding was a little stronger than anion-water type hydrogen bonding between H2O and the rest of the structure. When y was 1 or 2, Cl- and the plane of water molecule were parallel to the layer; while y was 3 or 4 , distribution of Cl- and water was random. Moreover, the LDHs-Cl-yH2O would change from ionic crystal to molecular crystal with the increase of number of water molecule. The hydration of LDHs-Cl would achieve a definite saturation state.