Abstract: In modern power grids, since large-scale grid connection of renewables is based on power electronic interface, the subsynchronous oscillation resulting from the interaction between power electronics and power grid becomes increasingly prominent. The wide-area monitoring system can obtain real-time data of sub-synchronous oscillations, which provides an important means for online analysis and early warning of sub-synchronous oscillations. In this paper, the basic framework of the subsynchronous wide-area monitoring system (SWAMS) was proposed to measure and analysis the subsynchronous oscillation. SWAMS was mainly composed of subsynchronous phasor measurement units (SPMU) and the subsynchronous oscillation analyzing center. Further, based on the combination of real-time monitoring data and aggregate impedance model, the stability evaluation methods and oscillation source-identifying were proposed. It can quickly and accurately realize online monitoring and early warning of subsynchronous oscillations. The effectiveness of SWAMS was verified by the simulation data collected from the electromagnetic transient simulation of a practical power grid of China.