Future power system faces several challenges，one of them is the high penetration level of intermittentwind power generation，providing small or even no inertial response and being not contributing to the frequencystability.The effect of shaft stiffness on inertial response of fixed speed wind turbines is presented.Four differentdrive-train models based on the multi-body system are developed.The small-signal analysis demonstrates nosignificant differences between models in terms of electro-mechanical eigen-values for increasing shaft stiffness.Thenatural resonance frequency of drive-train torsion modes shows slightly different values between damped andundamped models，but no significant differences are found in the number-mass models.Time-domain simulationsshow the changes in the active power contribution of a wind farm based on a fixed speed wind turbine during thesystem frequency disturbance.The changes in the kinetic energy during the dynamic process are calculated and theircontribution to the inertia constant is small and effective.The largest contribution of the kinetic energy is providedat the beginning of the system frequency disturbance to reduce the rate of the frequency change，it is positive for thefrequency stability.