The invention discloses a priority adjusting method and system for primary frequency modulation of a unit plant. The method comprises the steps that when a load command is increased, if the primary frequency modulation motion is positive adjustment, the load command is not locked, and a real-time load command is output; if the primary frequency modulation motion is negative adjustment, the load command is locked, and the current load command is kept; when the load command is decreased, if the primary frequency modulation motion is negative adjustment, the load command is not locked, and the real-time load command is output; if the primary frequency modulation motion is positive adjustment, the load command is locked, and a the current load command is kept. The method and system have the advantages that in grid frequency fluctuations, primary frequency modulation adjustment is preferentially carried out, the motion amplitude of the primary frequency modulation is stabilized, the phenomenon that due to the fact that the load command of a unit is different from that of the primary frequency modulation, attenuation happens to the real motion amplitude of the primary frequency modulation is avoided, the time for stabilizing the power grid frequency can be reduced, and stable and safe running of the power grid is maintained.