The operating safety of ultra-supercritical steam turbine is seriously affected by steam flow excited vibration in labyrinth seal. The DEFINE_CG_MOTION and DEFINE_PROFILE control macro were used to establish rotor whirl equation. The deformation of seal teeth under thermal and dynamic loads was calculated by Workbench fluid-solid coupling method. So the operating dynamic characteristics in seal were obtained by fast Fourier transform

and the rotor stability was analyzed. The results show that the steam flow can cause the deformation of seal teeth. The temperature made seal teeth length increase by 1%-1.5%

and the pressure and centrifugal effect had little influence on it. Under the action of thermal and dynamic loads

direct stiffness decreased

direct damping increased first and then decreased

cross-coupling stiffness decreased first and then increased

the maximum variation of dynamic coefficient was twice as before. Rotor stability margin decreased sharply in 35-55 Hz

so the rotor was more sensitive to the steam flow excited vibration. Due to the thermal and dynamic loads

the pressure fluctuations concentrated in low frequency range. The circumferential pressure fluctuations in seal could be increased by 18.5 kPa. The dramatic increase of pressure fluctuations in high pressure zone is the main reason for intense steam flow excited vibration.