Fault diagnosis of steam turbine is related to the safe and economical operation of the whole power plant. The fault diagnosis of thermal parameters can realize early detection and can quickly locate the fault position. In this paper

the diagnosis method of characteristic flow area and stage group efficiency based on on-site thermodynamic parameters is adopted to realize on-line monitoring and diagnosis analysis of unit flow state. Through fault diagnosis for a 300 MW unit

it is found that the efficiency and characteristic flow area of the stage group have changed obviously in the stage from main steam to the governing stage and stage from the inlet of intermediate pressure cylinder to the third extraction point. Based on the characteristics of governing valves

it is found that the change of efficiency and characteristic flow area from the main steam to the governing stage is caused by opening change of the high-pressure governing valves. The characteristic flow area reduces by 7.2%

and the efficiency decreases by 5.7%. Therefore

the fault is judged to have occurred at stage from the inlet of intermediate pressure cylinder to the third extraction point. The diagnosis results are accurate and completely consistent with the actual situation.