Abstract: With the continuous expansion of Guangdong power grid, the flow distribution of regional power grid is changing more frequently, which puts higher requirements on the voltage and reactive power coordination control of power plants connected to the system. The automatic voltage control system (AVC) is an important technical means to control the reactive power and voltage coordination control of control units in large power plants. It can maintain the safe operation of regional power grid, improve the system voltage quality, reduce the monitoring pressure of dispatch and power plant duty personnel, and frequently adjust the workload of reactive power, playing a positive role in the safe, economical and high-quality operation of the power grid. This article mainly introduces the control strategy and protection function of the AVC system in large thermal power plants. Combined with on-site AVC system debugging, testing, transformation, operation and maintenance experience, it provides reference for improving the regulation accuracy and qualification rate of AVC in large thermal power plants.