Abstract: Air path and changing state were explained in this article.From the thermodynamics point of view,this article suggests a concept of critical enthalpy,which means dehumidification efficient of wet air is zero with fixed refrigerating capacity and moisture content.Also,the indispensable condition of dehumidify efficient is greater than zero,and the ways to improve dehumidification efficient were pointed out in this paper.Therefore,based on the critical enthalpy concept, critical drying curve under various dew temperature was drawn on enthalpy-moisture figure of wet air.Area of dehumidification was divided,which establishing the indispensable basis for predicting dehumidification efficient and dehumidification capacity,also for further optimization dehumidifier operation.