Abstract: In this article, based on the large-scale off-design mathematic model, a thermal parameter fault diagnosis system is set up for nuclear power steam turbine. Due to the description of the mechanism of thermal power system in the mathematic model, the training procedure using large amount of operation data in former diagnosis system is avoided, which makes this method more practical. Because the method introduced the concept of Characteristic Parameter and diagnosis the faults by the relationships between the Characteristic Parameter and its Dominant Factor, the difficulty of fault diagnosis based on phenomena is also avoided. Further more the fault diagnosis result is quantitative and could be an ideal guide for the operation and maintenance of the object system.