The invention provides a power balance control method and system for replying a frequency voltage coupling effect. The method comprises the following steps: monitoring direct-current power modulationon a power grid after an alternating-current channel on-off failure occurs in the power grid; judging whether a low excitation limit action of an electric generator exists, and when a low excitation limit action signal of the electric generator is detected, adopting an uncoupling measure on the power grid according to a pre-established line set until no low excitation limit action of the electricgenerator exits. Through the technical scheme provided by the invention, an electric transmission line is uncoupled according to the low excitation limit action signal of the electric generator, the line charging efficiency is reduced, the power grid voltage and reactive power operation of the electric generator in a prescribed limit are guaranteed, and the safety level of equipment and the stableoperation capacity of the power grid are improved.