The automatic voltage control system of South Hebei Power Grid is a three grade coordinated control system including region ， province and area based on layering and zoning principles. The system gets control instructions from North China Power Grid and computes the voltage target values of power plant and substation according to the network loss minimized and the voltage qualified as a constraint, regulates the reactive power of the units and switches the capacitors and reactances of substation, and also sends the instructions to area dispatching. This paper introduces the main technical characteristics of the automatic voltage control system, analyzes the control effect and the significance in reducing network loss after closed-loop operation, based on actual operating experience, the countermeasures are proposed to the problems in operation.