Abstract: In order to improve the reliability of servo system, a triplex electro-hydraulic servo actuator system with three torque motors, three jet flapper valves and three sets of feedback components is designed. The authors make a deep theoretical analysis on the system, build up the mathematical model of the triplex electro-hydraulic servo actuator system, analyze the effects of the redundance control on dynamic performance of the system and the dynamic performance of triplex electro-hydraulic servo actuator system after any trouble happens. The results show when the triplex electro-hydraulic servo actuator system is applied, the dynamic performance has a little difference. When one coil of servo valve is broken, the system works in going order. This result proves that redundance control greatly improves the reliability of the servo actuator system, and provide the theoretical bases for the design of the servo actuator system with good performance.