The invention discloses a low-frequency oscillation monitoring method for a power system, and the method applies the oscillation feature identification method to carry out the preliminary identification of whether sampling data contains oscillation information or not, introduces the general digital signal processing method-digital filter method to extract related oscillation components, applies the wave peak indirect identification method to calculate oscillation amplitude through phase angle and amplitude value which are corresponding to each sampling value in a period wave and applies multi-level alarm thresholds to highlight important alarm information. The method can effectively improve the pertinence and the accuracy of the low-frequency oscillation on-line monitoring.