A periodical model for Mg3Al-LDHs-Cl-nH2O has been proposed. A geometry optimization and elastic constant calculation for the layered double hydroxides (LDHs) were carried out using the CASTEP/LDA (local density approximation) procedure at the CA-PZ level. The influence of interlayer water content (n) on the mechanical properties of the materials was investigated by analyzing the elastic constants Cij, shear modulus, Young's modulus, and Poisson's ratio, etc. Results indicated that the interlayer water content greatly impacted the mechanical properties of the materials. Interlayer water can enhance the compression properties of the overall system. When n=1, the compression properties of the material was best. When n=2, the capacity of the material to resist shear deformation was the worst and the system was most flexible. Interlayer water molecules greatly impacted the Young's modulus of the material while the impact on Poisson's ratio was not obvious. Interlayer water molecules played a horizontal role of averaging the mechanical properties of the materials. The compression performance and the expansion of the material in the x-axis and y-axis tended to be the same.