The measuring performance of the phasor measurement unit（PMU），especially under the dynamic condition，is the basis of deciding whether it can serve the power system dynamic monitoring，analysis and control.Based on abrief description of discrete fourier transform（DFT），the dynamic performance is analyzed and the averaging effect of DFT is revealed via the proposed phasor factor.Then，it is pointed out that，even in the phasor calculation averaging effect case，it is possible to reduce the dynamic measuring errors caused by the DFT averaging effect by changing the time tag position.The PMU experimental data are used to testify the theoretical analysis.Furthermore，in order to systematically and objectively evaluate the dynamic performance of the PMU，a PMU dynamic automation test system is designed in support of the PMU dynamic performance evaluation.