An integrated bidirectional motion piezoelectric stack electrohydrostatic actuator (PEHA) has been realized by integrating the Heidelfos thread insertion valve into the piezoelectric stack electro-hydrostatic actuator (PEHA). Based on the physical system of PEHA, the PEHA model has been established by using the Simscape. The experimental output flow and the simulated output flow of the piezoelectric stack electrohydrostatic actuator are compared under the conditions of different accumulator bias and different voltage peak conditions. The experimental output displacement and the simulated output displacement of the piezoelectric stack electro-hydrostatic actuator are also compared when the reversing valve period of the reversing valve is 4 s. The accuracy of the Simscape model is verified. Based on the verification of the system model, it is found that the backflow phenomenon of the valve plate is the cause of the attenuation of the output flow of the piezoelectric stack electrohydrostatic actuator at high frequency driving.