The hydraulically driven fatigue testing machine is studied to simulate the vibration in the fatigue experiments for the insulators. First, hydraulic scheme and control principle are introduced. A special composite cylinder is designed, which is controlled by a servo valve and a proportional relief valve to generate static and dynamic tensional force. Then, models of electro-hydraulic subsystems are analyzed. For the actual needs of the dynamic tensional force system, a high-fidelity force source is of great signi…