Abstract: The close relationship between activity coefficient and internal pressure is noticed. The van Laars theory is modified by substituting the internal pressure of van der waals fluid with Franks relation. An excess Gibbs free energy equation which is very similar to Wohls equation is derived and the famous van Laar equation, solubility parameter equation, Margules equation and Scatchard-Hamer equation etc. are all its special examples. The corresponding activity coefficient equation is established by further taking account of the contribution of excess entropy. It was proved that this equation is general, namely it is applicable not only to various small molecular liquid mixtures, but also to various polymer solutions, and has more extensive applicability than local-composition activity coefficient equations. In addition, a new definition of solubility parameter, which is more generalized than that proposed by Hildebrand, is obtained from this Gibbs free energy model.