Based on the analysis of the structure feature of PWR nuclear power plants,graph theory are introduced in the thermal economy analysis fields.According to the abstraction rule of the thermal system in PWR nuclear power plants,the boundary delimitation of a power plant thermal system is determined,and the thermal system of PWR nuclear power plants is expressed as the form of graph theory.A new unified rules for analyzing the thermal system are established.Combined with the first thermodynamics law and mass conservation law,weighted diagraph adjacency matrix is deducted.An example is given to illustrate the validity of the method.