Abstract: To solve the shaft vibration alarm problem of the main coolant pump in a nuclear power plant, the reason diagnosis and field verification are carried out. The methods of comparison analysis, spectrum analysis and orbit analysis have been applied to investigate the abnormal shaft vibration. The analysis result shows that the vibration of the pump shaft is greater than that of the motor shaft, and the vibration at the measuring points on the same horizontal plane in two different directions is similar. The abnormal vibration at Y direction of the motor shaft is caused by the failure of the cable shielding layer. It is suggested to use the soft materials, such as hemp rope for bandage in the vibration sensor cable installation. The abnormal vibration of the pump shaft is caused by large eddy motion and higher amplitude of running frequency components. In the case of the obvious vortex motion of the pump shaft, the shaft seal is suggested to be improved to reduce the vibration of the pump shaft.