There is vibration alarm which is caused by unstable shaft whirl in main coolant pump of reactor in many nuclear power plants. It is necessary to research deeply for the whirl characteristic. Firstly， the rotor dynamic of shaft in main coolant pump is analyzed. The frequency， amplitude and source location of whirl motion is measured and studied. The cause and the source of the whirl vibration for the shaft is investigated. The result shows that half whirl vibration frequency is slightly lower than 0.5 times the running frequency， and the whirl frequency vary in the range of 0.43~0.49 times the running frequency. As the amplitude of whirl motion is not constant， sometimes it is high， sometimes it is low. The divergent trend does not exist and the whirl is in dynamic balance condition in total. The fluctuation of vibration is mainly determined by the amplitude variation of half whirl frequency. The pump shaft whirl is caused by the water film whirl of the bottom bearing， not by the oil film whirl of the motor shaft. In order to reduce the influence of the shaft whirl， it is suggested to modify the bottom bearing of pump shaft.