The dynamic characteristics of a piezoelectric stack actuator for electro-hydraulic servo valve were analyzed. Based on the structure of piezoelectric servo valve discussed, the dynamic system model of piezoelectric stack actuator was established. Then, according to the transfer function of this model, the piezoelectric stack actuator was analyzed both in time domain and frequency domain. Moreover, the influences of damping coefficient, equivalent mass and stiffness coefficient of piezoelectric stack on the dynamic characteristics and output displacement of piezoelectric stack actuator were investigated through simulation. The simulation results show that the dynamic characteristics and output displacement of piezoelectric stack actuator can be increased by increasing the damping coefficient, reducing the equivalent mass and increasing the stiffness coefficient of piezoelectric stack. The results provide theoretical basis for the structural parameter optimization of piezoelectric stack actuator for electro-hydraulic servo valve. 