Abstract: Based on the wide area measurement information, a fast prediction method for coherent generators based on short-time voltage phase trajectories was proposed. Firstly, based on double machine oscillation model, the geometric characteristics of bus voltage phase trajectories were analyzed, and an identification method for coherent generators was proposed. Secondly, based on the variation trend of voltage phase trajectory, the coherency of the generator groups was identified, and the extended equal area criterion (EEAC) was used to further estimate the transient stability margin, which can verify the identification results. Finally, in order to deal with the problem that the generator coherency is not obvious in the initial stage of the out-of-step oscillation, a prediction scheme of coherent generators based on sliding time window was proposed. Continuous monitoring and real-time checking of the generator coherency were realized by multiple clustering and checking. The proposed method was validated with the IEEE-9, IEEE-39 and IEEE-118 bus system.