The electro-hydraulic actuator (EHA) can achieve fault-safety without mechanical jamming and other faults,and has obvious power weight ratio advantage for high power applications.Therefore,it can be applied to servo control system of aircraft main control surface.EHA composition,working principle and the function of functional components were introduced.The mathematical model of the EHA servo system under three loop closed-loop control was derived in detail,the frequency domain analysis was carried out and the servo parameters were designed.The simulation model of the EHA servo system was built in AMESim.The derived EHA mathematical modeling method can be used to design the parameters of EHA servo circuit and analyze the sensitivity of key parameters.The simulation method contributes to verify EHA servo system performance at design beginning and provides theoretical basis for EHA physical design.