Abstract: Considering the thermal economic analysis methods of PWR nuclear power plant,a Steam-water Distribute State Equation has been established and a Matrix for calculating the efficiency is derived by the aquivalent enthalpy drop theory,the theory of matrix and the normal thermal-equilibrium method,suitable for quantitative thermal efficiency analysis of secondary-circuit of PWR nuclear power plant.The structure of this matrix has a mapping relationship with the secondary-circuit of PWR nuclear power plant,and it can simplify the thermal economic analysis of PWR nuclear power plant,and provides a theoretic principle for analysing the economics of the thermal system of the secondary-circuitof PWR nuclear power plant.An example is given to illustrate the validity of the method,and it indicated that the thermal economics diagnostic method is well defined and easy to be used in system design and operation diagnosis.