A new nonlinearity, instability and non-stationary signal processing method named improved Hilbert-Huang transform was proposed to analyze the measured oscillatory signal from wide area measurement system. Based on this method, the mode mixing of the measured signal in decomposition process was removed and the scheme has advantages of good performance of anti-interference. The measured signal was decomposed into a series of models and identified the amplitude, frequency and damping of lowfrequency oscillation signal. The measured signals from simulation model and actual power system indicate the validity and feasibility of the proposed method.KEY WORDS：Hilbert-Huang Transform (HHT); lowfrequency oscillation model; power system; online identification I.