Abstract: The thermodynamic swelling characteristics of chitosan membrane and H_2SO_4 crosslinking chitosan membrane in water-ethanol mixture were studied.Thermodynamic swelling distribution relationship showed there existed strong coupling effect in the themodynamic sewlling process.In order to analyze the coupling effect in the thremodynamic swelling process,the relationship between the activity and volume fraction of each component within the membrane was defined.And the concept of reference state in the thermodynamic swelling process which only considered the interaction between individual component and membrane matrix was also set up.The interaction between H_2O and the membrane matrix dominated the activity coefficient of H_2O within the membrane,But the activity coefficent of EtOH was dominated strongly by the coupling effect between H_2O and EtOH within the membrane because of existence of H_2O within th membrane