The electro-hydraulic servo and actuator system is one of the most important components in prime mover governing system. Precision and adaptability of the math model of the system significantly affect validity of power grid stability analysis. It is found that the existing models of the electro-hydraulic servo and actuator system can simulate the power grid performance properly when valves have small actions. However these models can not exactly reflect practice in case of large valve actions， so that they can not be used to analyze power grid stability while valve acts in large range. A new math model of the electro-hydraulic servo and actuator system is proposed based on operating principle of each component. This model is capable of evaluating the power grid performance at both small and large valve actions with one set of model parameters. Method of determining these parameters is provided， and the new model is verified by corresponding experimental data from the site.