The invention discloses a PMU and a method for achieving the on-line identification and alarm of the secondary synchronous oscillation. The PMU comprises an anti-aliasing filter module, an ADC sampling module, a synchronous sampling clock module, a DFT quadrature conversion module, a front-loading filter module, a phasor amplitude analyzing and processing module, and an uploading and processing module. The method comprises the steps of accurately calculating the synchronous phasor; pre-filtering the synchronous phasor; sampling and processing the calculated amplitude of the phasor; analyzing the frequency spectrum of the phasor amplitude; identifying the secondary synchronous oscillation according to the components of the frequency spectrum of the phasor amplitude; uploading the frequency components of the secondary synchronous oscillation and the alarm information to a dispatching master station. According to the technical scheme of the invention, the on-line identification and alarm of the secondary synchronous oscillation of the power system can be completed by the PMU. Through adopting the analysis on the frequency spectrum of measured high-density phasor data, the frequency components of the secondary synchronous oscillation can be accurately calculated. Therefore, the analysis monitoring accuracy and the analysis monitoring adaptability of the secondary synchronous oscillation are improved.