Correct judgment of the oscillation mechanism has a great significance in rationally selecting the oscillation suppression measures and quickly suppressing the oscillation when a grid low-frequency oscillation occurs.Based on the actual measured data of wide area measurement system（WAMS），this paper analyzes the characteristics of the negative damping oscillation and the forced power oscillation in the pre-oscillation，the transient stage，the steady-state stage，and the decay stage，and studies the method of discriminating the negative damping oscillation from the forced power oscillation in different stages.Through the comparative analysis of two actual cases occurring in the grid，the conclusions of the theory analysis are verified.