Aimed to solve the problem--power systemSSlow frequency oscillationSin area main lines,Sthis paper presents aSdecomposition of power systemSlow frequency oscillationSwhich canS monitorSvibrationS modal parametersSquickly and accurately;SThe coreSof low frequency oscillationSmonitorSis an improved Hilbert-Huang transformS(Hilbert-HuangStransform,SHHT)Salgorithm,Sthe algorithmSthrough the onlineSrecord data superposition integral cycle cosine both ends of the signal,Ssignal preprocessingSis the extreme point. The method has solved the EMDSdecompositionSprocess of "end effect" problem;Sthrough the MATLAB simulationSverifies the effectiveness of the proposed method.