To better understand the dynamic characteristics of electric pitch system in a wind turbine generator system（WTGS），it is necessary to study its dynamic model with consideration of drive motor characteristics.By introducing the basic principle of the electric pitch system，its detailed models are presented.The model includes the subsystem models of three-phase induction motor and its vector-control strategy，driver train，back-up battery.A simplified first-order inertia model for the electric pitch system is also introduced.In order to validate the detailed model，a test platform for the electric pitch system is constructed.And additionally，the dynamic performances of the detailed model，including the pitch angle tracking performances，the load disturbance performances，and the low voltage fault performances，are simulated.The simulation results are also compared with those of the simplified first-order inertia model and those of the test platform for the electric pitch system.The comparison shows that the proposed model can better reflect the transients of a real electric pitch system，and can also be used to evaluate the operation characteristics of a real electric pitch system under low voltage faults.