The instrument transformer is the necessary equipment which connects the primary power system and PMU. Therefore, the measurement accuracy of PMU is directly affected by the transfer accuracy and the transient response of instrument transformer. In this paper, the dynamic signal models are abstracted from the phenomena of practical power systems. Under such kinds of dynamic signal models, the impact of instrument transformer on PMU is analyzed in detail, especially for the saturation problem of the current transformer and the ferromagnetic resonance and transient response of capacitor voltage transformer. Based on the analysis, the suggestions for the evaluation criteria of PMU dynamic behavior can be provided. Moreover, the principles of choosing instrument transformers in the wide-area measurement systems are suggested as well.