The invention discloses a power grid voltage control method. The power grid voltage control method is used for controlling a voltage of a 500kV bus of each substation near region in a 1,000kV ultrahigh-voltage system after the 1,000kV ultrahigh-voltage system runs and comprises the steps of analyzing voltage fluctuation of 1,000/500kV after a fault under a normal running mode of the 1,000kV ultrahigh-voltage system and a maintenance mode of an ultrahigh-voltage component to acquire voltage rising values of the 500kV bus of each substation near region before and after the fault; and acquiring a voltage control upper limit value of the 500kV bus of each substation near region in the 1,000kV ultrahigh-voltage system according to the voltage rising values of the 500kV bus of each substation near region before and after the fault and by combining a voltage early-warning upper limit value of the 500kV bus of each substation near region, and controlling the voltage of the 500kV bus of a substation near region corresponding to the voltage control upper limit value. The invention also discloses a power grid voltage control system. By the power grid voltage control method, the reactive power regulation capability of the 1,000kV ultrahigh-voltage system can be effectively improved, and the 500kV power grid voltage is effectively controlled to the early-warning upper limit value or below.