Abstract: Three constant speed pumps with 33.3% rated capacity are used in the main water supply system of a nuclear power project. During the commissioning，drive shaft end bearings of all the main feedwater booster pumps were of excessive vibrations，which grew slowly with the increase of running time. To solve the problem，we took a series of measures，such as re-alignment of the coupling，fastening inlet and outlet pipe supports，readjusting the parallelism and concentricity of pipes and adjusting the bearing interfaces，but none of them can effectively alleviate the vibration.Through analysis on the vibration spectrum ，as well as the manufacturing and installation features of the pumps，we have formulated and implemented a bearing box transformation scheme for the main feedwater booster pumps，which has greatly reduced the vibration value，effectively solved the problem of excessive vibration，and improved the stiffness of pump body and bearing case.

Key words: nuclear power plant, main feedwater booster pump, excessive vibration, bearing box transformation, spectrum analysis, stiffness