［［Objective ］］ To propose a method for modifying the existed isotherm model based on molecular thermodynamics.［［Methods ］］ The phantom model in polymer physics field was borrowed to predict the chemical potential of water from swelling effect.Isothermal adsorption and desorption of wheat flour measured using a dynamic vapor sorption system at 20，30，40 ℃ were used as a case to verify our improving approach after analyzing the reasonability of best fitted parameter values.［［Results］］ The modified model can describe the adsorption and desorption of wheat flour very well,give reasonable parameter values,and predict logically the variation of bound water and adsorbed water contents with water activity and the contributions of mixing,swelling and adsorption effects to the chemical potential of water.［［Conclusion ］］ Our approach for modifying the molecular thermodynamic isotherm model is effective and reasonable.