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An online evaluation method is proposed for PSS in this paper, which is an advanced analysis function based on Wide Area Measurement System (WAMS). It tracks the operating status of the power system in real time. Once low-frequency oscillation is detected, the developed evaluation function first collects the control output of the PSS to be assessed and the various recording signals reflecting the system oscillation, for instance, the power of the generators and important tie lines. Next these data are analyzed and the parameters of the oscillatory modes are computed using Prony algorithm. Then the phase relationship between the power deviation generated by the PSS control and power oscillation actually measured is analyzed for every modes, in order to provide a quantitative estimation criterion for the effectiveness of damping low-frequency oscillation of a certain mode. Theoretical analysis and simulation was performed to validate the correctness of the method proposed.