Abstract: Aiming at the abnormal low-frequency vibration of the three-bearing reactor coolant pump during the service period of the power station, the ANDRITZ reactor coolant pump was taken as the research object. Based on the calculation methods of fluid dynamics and rotor dynamics, the vibration characteristics of the reactor coolant pump shafting, the generation mechanism of half-frequency whirl of the reactor coolant pump, the key factors affecting the stability of water-lubricated bearings and their physical nature were explored. Then the water-film whirl problem of reactor coolant pump was studied. It is found that the nonlinear dynamic instability of water-lubricated bearing induces the half-frequency whirl phenomenon of reactor coolant pump. Under rated working conditions, the amplitude boundary of the nuclear pump shafting is double horn type, and the center point of the double horn is near the bearing area of the drive end of the motor. The vibration characteristics of the reactor coolant pump shafting in different coaxial segments are significantly different. The half-frequency whirl phenomenon mainly appears in the area near the coupling and the pump shaft side, while the upper shaft section of the motor is mainly power-frequency vibration, and the impeller side shaft section is mainly half-frequency vibration. The tangential force perpendicular to the rotation direction of the rotor is the key force leading to the water-film whirl problem, and the cross-stiffness coefficient and the average circumferential velocity ratio of the liquid film of the water-lubricated bearing are the key physical quantities affecting the tangential force. The research results are helpful to the optimal design of reactor coolant pump, fault diagnosis, and operation and maintenance management of power station.