Abstract: The user-defined function of FLUENT and the relative rotation model were used to simulate the semi-conical whirl motion of 1000MW rotor. The characteristics of steam flow excited vibration in the seal were explored. Meanwhile, the dynamic characteristics were obtained through the fast Fourier transform, and the dynamic seal-rotor stability in the semi-conical whirl motion was analyzed. The results show that the dynamic coefficients fluctuate significantly when the rotor whirls in semi-conical orbit. The absolute values of k_zz and k_yy are four times that of parallel whirl motion. The direct damping c_zz and c_yy have the opposite variation. The cross-coupling stiffness k_zy and k_yz are decreased, and the working effect of exciting force F_z is increased in the tangential direction. The relative change of cross-coupling damping is less than 35% after 25Hz. With the increase of whirl frequency, the effect of semi-conical whirl on the seal-rotor stability increases significantly, which is not conducive to the rotor stability. The increase of the turbulence effect, the change of teeth-top jet flow and the evolution of vortexes in space will aggravate the pressure fluctuation and unevenness in the seal, which enhance the influence of steam flow excited vibration and lead to the decreasing of rotor stability.