Abstract: An optiniized design theory of electro bydraulic force-applying system is. Presented in this paper in which are: 1) System optimization: Electro-hydraulic force-applying systems are divided into thrree types-follow type, containment type and sinusoidal type, and the optimized design methods are given for each type;2 ) Element optimization; Here a P-Q calculating chart is introduced which may be used for selecting the optimum values of the load-free flow of the servo valve and effective cross-sectional area of oil-cylinder; 3) The principle of structure invariability is given, which can be applied to eliminate the intense disturbance, load effect and the cross linking effect; 4) Two kinds of operating conditions and perfect load curves of servo valve are introduced in the paper in order to discuss the process of occurrence of the disturbing forces and the relationship between the forces and the open center of the servo valve.