The invention discloses a method for realizing automatic voltage closed-loop control of a power plant by taking a powder factor as a control target. The method comprises the following steps of: 1) acquiring the analog quantity information and switching quantity information of an electrical control system of the power plant in a communication mode; 2) according to an actual power factor and a target power factor of wire outgoing of the power plant, calculating total reactive power required for the power plant; 3) reasonably allocating the total reactive power to each generator set by calculation; and 4) sending magnetizing and demagnetizing signals to an excitation system of a generator to regulate reactive output of the generator, so that the power factor of wire outgoing of the power plant reaches a control target value, and the voltage reactive automatic control of each generator set of the whole power plant is realized. By adoption of the method, a closed-loop control requirement for reactive power balance between supply and demand in the system of the power plant is completely met. The method has the characteristics of high self-balancing capacity, high generality, simplicity and intuitiveness.