To investigate the rotor-bearing dynamics and solve the unstable synchronous vibration problem under partial admission conditions in axial steam turbines, a three-dimensional computational fluid dynamics model of the governing stage for a 350 MW turbogenerator unit was established. Steam force under different partial admission cases was calculated. Results show that the uneven steam force in the horizontal direction was fairly as strong as that in the vertical direction. The maximal horizontal steam force was about 40% of the high intermediate pressure cylinder rotor weight. Synchronous vibration faults caused by partial admission may occur in practice. The finite element model was further built to analyze the rotor-bearing dynamics under partial admission conditions. Large fluctuation of the synchronous vibration was observed during the variation of control valve sequence. Finally, the effect of bearing elevation on the rotor-bearing dynamics was studied. The sensitivity of bearing load varies for different bearing elevations. An effective solution to the unstable synchronous vibration faults caused by partial admission was achieved by optimizing the distribution of bearing elevation. A relevant case was presented through practical test.