The daily statistics of valid disturbances recorded in the historical data of the wide-area measurement system (WAMS) can reflect the stability level of a power grid. The formulas of the small signal analysis including power disturbance on the grid is given firstly; then the relationship between damping ratio and response of the oscillation magnitude of the tie-line active power is analyzed; and the relationship of the oscillation probability and damping ratio is obtained. Finally, a case study based on 10-machine 39-bus power system verifies the validity of the expression of oscillation amplitude of tie-line active power and the relationship between the oscillation probability and damping ratio. These theoretical results can be used to guide the power operator to assess the damping level of the power grid based on the daily oscillations number recored by WAMS.