Thermodynamic approach is applied to energy utility of wasterater treatment process, definition and calculation of pollutants' chemical energy would be primary. According to the First Law of Thermodynamics, the concept of pollutants' chemical enthalpy advanced, and criterion for calculating chemical enthalpy is integrated, and enthalpy calculation method of complex organic substances established too in this paper. Then enthalpy was coulped with traditional water quality index-COD. Energy analysis conclusions of various processes in wastewater treatment and different classification energy can be comparable through the study, energy index becomes easy to be linked up with traditonal water quality mensuration.