As a most important part of DEH (Steam Turbine Digital Electro-hydraulic Control System), EH system is composed of oil supply system, actuator and emergency trip system. The oil supply system is a center to store and deal with EH oil and it can actuate servo system by supplying the stable high pressure oil to EH system. Actuator can adjust the opening size of each steam valve by responding to the electro-instruction sent from DEH. Emergency trip system is controlled by the trip parameter of steam turbine. When these parameters exceed the limit value, the trip system will shut off all of the entrance steam valves or only the adjust steam valve in order to ensure the secure running of the steam turbine.