Abstract: After the ultra supercritical boiler of a power plant was put into operation,the bearing vibration of the primary fan was abnormal,which seriously threatened the safe and stable operation of the boiler system. Through the simulation calculation of the operating conditions of fan blades and the flow field in the outlet air duct,it is diagnosed that the vibration is caused by the poor matching degree of the relative angle between the moving blade shape and the guide vane and the poor quality of the flow field in the outlet air duct. The scheme of changing the blade type and optimizing the flow field of the outlet air duct of the fan are put forward,which solves the problem of longterm abnormal vibration of the bearing of the primary air fan in the power plant.