Based on phasor measurement unit（PMU）measurements and by applying correlation analysis of oscillating components of excitation current and rotor speed（frequency），an online analysis method is proposed for evaluating overall damping characteristics of excitation system（including main control and additional control）in the process of low frequency oscillations. The method is independent of the accuracy of excitation system and its control systemsmodels. Actual examples recorded by wide area measurement system（WAMS）show that，the method proposed can verify the rationality of the excitation system and its control parameters，and provide reliable reference for the disturbance source location analysis and control of negative-damping low frequency oscillations.