The invention relates to an optimization control method for reactive power of a power plant. The method comprises the steps of controlling reactive power of a power generator with an automatic voltage control device by taking economic operation as an objective, and automatically controlling the reactive power of the power generator by taking examination voltage and a power factor of the power generator as limiting conditions. The method reduces copper loss, iron loss and stray loss of the power generator based on meeting a reactive power demand of a power system, and allows the power generator to perform energy-saving and economic operation.