The invention discloses a wind power plant-power grid interaction modal identification method and system. The wind power plant-power grid interaction modal identification method comprises the steps: carrying out the decomposition of wind power plant power data through employing a rotation invariant subspace algorithm, and obtaining a signal subspace; performing Hilbert-Huang transform on the signal subspace to obtain a frequency set; and comparing the frequency set with a preset characteristic frequency range, and determining a wind power plant-power grid interaction mode. According to the wind power plant-power grid interaction modal identification method, the signal subspace is separated from the noise subspace, so that the problem that a conventional signal processing method is greatlyinfluenced by noise is solved, wherein each modal component obtained through empirical mode decomposition of the signal subspace contains local feature information of different time scales of the original signal; and by comparing the instantaneous frequency of Hilbert-Huang transform with a preset characteristic frequency, the type of the wind power plant-power grid machine-network interaction isobtained, and the wind power plant-power grid interaction modal identification method is suitable for processing wind turbine generator data with randomness and confusion.