Abstract: With the rapid development of wind generation, the characteristic of time varying amplitude and frequency of sub-synchronous oscillation is more and more exhibited due to the stochastic fluctuation characteristic of wind generation. A sliding window FFT based monitoring method is proposed for dynamic identifying the time-varying oscillation modes. The method proposed in this paper is more practical and with anti-noise ability. This monitoring method could eliminate the frequency aliasing even if the oscillation frequencies are close to each other and could realizes the on-line monitoring and analysis of stochastic time-varying sub-synchronous oscillation modal.