Abstract: The instability of primary frequency modulation of steam turbine units is not rare, and it affects the security of power grid operation seriously . The study on the specific case of East China power grid figures out the root cause of primary frequency control capacity instability of being the large turbine local droop. After analyzing the main factors influencing the turbine local droop, it is concluded that the turbine local droop is closely related to the corresponding power grid frequency deviation. A 600-MW supercritical steam turbine is taken as a case to generate the optimized local droop curve. The results can be used to enhance the capability of the turbine and power grid coordination of steam turbines.