Abstract: There always presents the shaft train periodic vibration fluctuation phenomenon at unit 1 in a nuclear power station from its debugging time, the maximum fluctuation swing are at LP2 rotor No5, No6 shaft position, this paper summarize the vibration features and rule by a long time tracing and investigation, and by means of a deep research of the vibration mechanism combining with the unit structure feature and the vibration responding character of congeneric units, then point out the fault position by theoretical calculation at the end, and presents that the root cause of the shaft train periodic vibration fluctuation is that the exciter side seal ring of generator suffering Morton Effect, then arise a solution, this problem which confusing station many years was solved at last. The analysis process and solution described in this paper have a reference meaning for those lager units that have similar vibration problem.