Abstract: Taking the PG9171E gas turbine as the top cycle unit as the research object, the gas-steam combined cycle system model for varying operation conditions is established by using the EBSILON software. The influence of the extraction rate variation on the peak-shaving capacity of the combined cycle is analyzed by changing the steam extraction and boundary conditions. The results show that the peak regulation margin of the unit is 32.16~213.65 MW when the steam extraction is lower than 135t/h. The minimum and maximum generation power of the combined cycle unit with different extraction capacity are compared in this paper. It is found that the minimum generation power increases and the maximum generation power decreases with the increase of the extraction capacity. The results can provide an important reference for the economic operation of combined cycle units and the decision of power grid peak-shaving.