Abstract: Power system online mode estimation is the best way to observe real-time oscillations. In this paper, we propose a novel online mode identification formula, which firstly applies the results from modal analysis to online mode identification and so overcomes the defect of measurement-based methods that results from them cannot be directly used to damp oscillations. In this formula, a Prony-based multi-signal processing (PMP) method is developed to estimate the mode shape from ringdown data and a mode shape matching method is firstly introduced for finding out which modes are active in the real-time oscillation of a large-scale power system. Simulation studies based on IEEE68 system and China Southern Power Grid have been carried out to verify the correctness and applicability of the proposed formula and methods.