In order to achieve online flywheel moment monitoring, formulas to calculate the flywheel are derived and research on the flywheel moment is performed based on the increase in speed in the load rejection mode of a hydro-generator unit. A new method is proposed to monitor the flywheel moment. Speed signal sources are sorted into analog signals from the control system and signals from the toothed disc. For the analog speed signal, the wavelet theory is adopted to de-noise the signal, then the correlation coefficient between the speed and defined time is calculated to determine the effective increase in speed within the defined correlation coefficient. For the signal from the toothed disc, a maximum correlation coefficient between the speed and the limited period of time is calculated to determine the effective speed signal. Finally, the flywheel moment is calculated based on GB/T1029—2005. The effectiveness of this method has been tested and verified through case study.