Abstract: For the No.3 bearing with large amplitude vibration in a 25 MW turbine generator, the problem of abnormal vibration was not be solved after maintaining in several times, which resulted in the shutdown of the turbo-generator set. Through analyses on the operation parameter, vibration spectrum, bearing structure and maintenance process of the turbine generator, it is confirmed that the main reason of large amplitude vibration is the change of centering, which is caused by the friction between the sinking generator rotor and the shaft seal and oil shield with the loosening of support system due to the false connection of the supporting bracket at bearing bottom. By means of optimizing the centering adjustment process for rotor and adjusting the assembling clearance for bearing, the vibration amplitude of No.3 bearing is reduced significantly, which can guarantee the safety and stability of the unit operation. The reasonable maintaining and assembling method for rotor is vitally important for the reliable operation of turbine generator.