Abstract: In a nuclear power plant, the vertical pump motor has a large vibration difference between the two radial directions, and the vibration phenomenon slowly rises after the start. To solve the vibration problem, this paper uses methods such as spectrum analysis, natural frequency analysis, and phase analysis to diagnose the fault. It is found that the matching motor is with the coupling problem of structural resonance, rotor thermal bending and dynamic unbalance, and the vibration problem is solved by adjusting the bolt and the on-site dynamic balance method.