Abstract: The heat efficiency online monitor and diagnosis system is used in nuclear power unit for the online monitoring, calculating and trending the efficiency of the thermodynamic system. It’s able to timely detect the events and diagnose the equipment caused heat efficiency reducing. And the system can take a timely and efficiently technical support to the operators and maintainers. The study on the design of heat efficiency online monitor and diagnosis system for nuclear power unit was introduced. The technical criteria as well as the solution of the system were expounded. The designed system can satisfy the real-time online monitoring, calculation and trend tracking of thermodynamic loop of nuclear power plant, timely detect heat efficiency deterioration events, and diagnose and locate the equipment caused heat efficiency deterioration.