The principle and structure of the online system of failure diagnosis for the superheater and reheater in utility boilers is introduced. The realization method of several main modules of the system is also presented. With this system the simulating calculation of wall temperature distribution of pipes system of superheater/reheater can be carried out and can be used to forecast the local situation of overheating of pipe materials. The system is helpful to raise the safety and economy of power plant.