Abstract: Combined with thermal system characteristics and actual operation of nuclear power steam turbine, different methods for calculating the enthalpy of the wet steam are studied. According to the turbine manufacturer design data and unit operation data, based on the method of assumption of wet steam dryness, the calculation model of the operating economy indexes of turbine-generator set is established. Through on-line continuous monitoring of nuclear power plant operation economy indexes, and the analysis of operation performance data of thermal system and equipments, the method of fault diagnosis for thermal system is studied. It can provide technical measures for finding and solving the energy loss problem in the actual operation of the secondary circuit system.