The dynamic characteristics of phasor measurement unit is the important base of the reliability of advanced applications of wide area measurement system. The paper proposes a PMU dynamic characteristics evaluation method based on instantaneous value comparison during featured dynamic period of great disturbances. The method restores phasor data to instantaneous sampling data, obtains real sampling data from fault recorder, and analyzes the differences in correlation, response speed of phasor algorithms, amplitude change and total similarity between instantaneous sampling data from phasor and instantaneous sampling data from fault recorder. The PMU dynamic characteristics can be roundly and quantificationally evaluated by the indexes. The theory has been proved correctly and feasibly by practical-environment PMU data. Furthermore, for practical application, the paper designs an online evaluation system, which provids realtime monitoring and systematic evaluation.