Since non-stationary signal of hydro-power unit can be analyzed in details by use of continuous wavelet transform（CWT）， implicated information about the operational status can be extracted in the scale domain using the CWT. Power rate of each frequency is proposed to be refined to deal with the CWT coefficient figure's pixel brightness. Gray moment is then introduced to quantitatively indicate the characteristics of the CWT coefficient figure. It is demonstrated in the paper that the gray moment is effective to be a hydro-power unit vibration symptom for the hydro-power unit analysis and diagnosis.