The Gibbs distribution is introduced into the theory of polymer solutions．The statistical mechanics method is used to deduce the thermodynamic theory of polymer solutions．The analytical expressions of thermodynamic quantities

including the free energy of mixing and the entropy of mixing

are presented in the present theory．The free energy of mixing has three fundamentally different types of contributions

i．e．

the translational free energy

the configurational free energy and the thermal correction free energy. The first two terms of the free energy of mixing are equal to the corresponding terms in the FH theory．The influence of interactions among chain units and solvent molecules on the configuration is included in the third term．In the case of small interactions among solvent molecules and chain units the expression of free energy of mixing in the present theory approximates that of the FH theory．An apparent Flory interaction parameter χAP is introduced in this paper. It not only depends on temperature and the interaction energy as the Flory interaction parameter χ

but also depends on the concentration of polymer solutions．The difference between χAP and χ increases

as the interaction energy increases or the concentration of the solution increases.