A Siemens 9F gas turbine had vibration problem of No. 6 and No. 8 bearing since its construction, which seriously threatens the safe and stable operation of the unit. The vibration of NO.8 bearing suddenly increased during the engagement or release of the SSS clutch, which was mainly low frequency component, and was diagnosed as the oil whirl. Due to the structure design, installation conditions and operation conditions, which results in low supporting stiffness of NO.8 bearing. The vibration of NO.8 bearing increased obviously in cold start-up, which was close to the jumper value. After a series of measures and attempts such as raising the bearing elevation, reducing the bearing length, adjusting the cat-claw load of middle-low pressure cylinder and so on, the problem of abnormal vibration of SIEMENS 9F gas turbine shaft system has been solved thoroughly, which provides reference for similar vibration problems.