Abstract: Based on the running experience for 100 D Main Coolant Pump, its typical abnormal vibration phenomena is studied, and a vertical rotor-bearing system dynamics theory is established to analyze the vibration characteristics of the rotor. It can conclude that as for the conference of vertical construction, three bearing supporting structure and watered guide bearing, when the running condition is stable, the vibration state is stabilized, when the running condition is influenced by the circumstance disturbance or in transient state, the vibration will be instabilized and the vibration trend will change in a nonlinear character. In order to maintain a stable running condition,a fine balance test should be done to reduce the vibration value of the main coolant pump as low as possible.