This paper introduces the composition of electro-hydraulic servo control system. Especially, the triangular wave generator, PWM principle and the power amplifier stage circuit in the servo amplifier are described. Finally, the frequency response of servo valve to the PWM driving signal is analysed. 在此介绍了电液伺服控制系统的组成,对伺服放大器中的三角波发生器、PWM调制原理和功率放大级电路进行了详细说明,并分析了伺服阀对PWM信号的频率响应特性。