The invention discloses a frequency modulation method and device for a steam turbine generator unit, and relates to the technical field of thermal power generation. A frequency modulation instructionis received through analysis, and the frequency modulation amplitude value and frequentness carried in the frequency modulation instruction are obtained; according to the frequency modulation amplitude value and frequentness and a preset mapping relation tablet, ID of valves and the opening degree corresponding to the ID of each valve are found out; and finally the current opening-closing state ofthe valves arranged between a steam cylinder of the steam turbine generator unit and a heater and associated with the ID of the valves are controlled to be switched into the corresponding opening degree, and thus the steam extraction capacity discharged to the heater by the steam cylinder can be changed. Understandably, when the steam extraction capacity is large, the power generation power of the steam turbine generator unit is small, when the steam extraction capacity is small, the power generation power of the steam turbine generator unit is large, thus by changing the steam extraction capacity discharged to the heater by the steam cylinder, the power generation power of the steam turbine generator unit is changed, power grid frequency modulation is completed, the response speed is high, the construction cost is low, and the application range is wide.