The invention discloses a PMU-based primary frequency modulation correction method and system for a power grid cycle change rate. A power grid synchronous frequency phasor processing unit converts received PMU-based Hzi into a change rate of a synchronous frequency phasor, then determines whether the change rate of the power grid synchronous frequency phasor exceeds a power grid cycle large disturbance threshold or not, and determines whether the power grid synchronous frequency phasor exceeds the power grid cycle large disturbance threshold or not, so that whether the power grid frequency haslarge disturbance or not is determined in advance. If the power grid frequency is confirmed to generate large disturbance, the primary frequency modulation correction system adopts a control mode of'shortening the dynamic stabilization time of the output of the unit and increasing the compensation electric quantity' for the large disturbance, and the requirements of the power grid primary frequency modulation checking and evaluating system on the large disturbance calculation are met. The system and the method can ensure that the qualification rate of the primary frequency modulation actionof the unit reaches 100 percent, and the illegal response times of the primary frequency modulation of the unit are zero, thereby improving the safe operation of a power grid and the unit.