To explore the dynamic characteristics of the rotors of labyrinth seal considering inlet preswirl,based on Murphy’s small displacement principle,the governing equations of steam flow excitation force and rotor vortex displacement were established,and a multi-frequency elliptical vortex model was built by user-defined functions.The exciting forces of three labyrinth seals,straight tooth labyrinth seal(SLS),inclined tooth labyrinth seal(ILS),stepped tooth labyrinth seal(STLS) were numerically solved under different inlet preswirl ratio,the seal dynamic characteristic coefficient was obtained by fast Fourier transform,and the stability of the rotor and the leakage of the seals were analyzed.The results show that the direct stiffness coefficients of the three labyrinth seals decrease first and then increase in the range of eddy frequency from 40 Hz to 180 Hz,showing a strong frequency correlation.While the influence of sealing structure and inlet preswirl on damping coefficient can be ignored.The damping coefficient slowly decreases to a negative value and then becomes stable with the increase of vortex frequency.STLS has the smallest damping coefficient and the worst stability,while ILS has the best stability.Increasing the inlet preswirl ratio will reduce the effective damping coefficient and lead to a decrease in leakage rate.STLS exhibits the best sealing performance under different preswirl conditions,with a 15% and 7% reduction in leakage compared to SLS and ILS,respectively.The cross stiffness of the three labyrinth seals increases from negative to positive in the studied vortex frequency range of 20 Hz to 200 Hz,while the effective damping decreases to negative.Therefore,the stability of the rotor system of the labyrinth seals is poor,which may induce rotor instability.