Abstract: A perturbation model for electrolyte solution has been developed. A mixture of charged hard spheres and neutral hard spheres is used as the reference system of perturbation expansion and the interactions between various particles including repulsion force, dispersion, induction and electrostatic force are taken as the perturbation terms. The expressions of Helmholtz free energy and internal energy are deduced. Comparing with the methods in the literature, the reference system and expansion of perturbation terms of our model are more reasonable. The long - range term is simpler and the effect of the size difference between particles can be partly shown. The internal energy results calculated with our perturbation model are close to MC data.