The invention discloses a PMU data processing method and device for low-frequency oscillation online analysis, and belongs to the technical field of a power system and automation thereof. After PMU real-time data of each monitoring device is read through a low-frequency oscillation monitoring and analyzing function of a WAMS master station and before low-frequency oscillation monitoring and analysis is carried out, abnormal data is screened according to the change quantity between data points and rated power of each device, then, the abnormal data is repaired, and filtering and sampling are carried out on the PMU data, thereby ensuring calculation efficiency and calculation accuracy of low-frequency oscillation monitoring and analysis.