Abstract: The flow rate and system resistance are not matched with the characteristics of the induced draft fans in thermal power units under low load condition for deep peak regulation, leading to the vibration of fans is more sensitive to external disturbance and its own state change. To solve the problem that the vibration exceeded the limit of an axial flow induced draft fan with adjustable rotor blade under low load condition, according to the vibration characteristics and the feedback effect of vibration treatment, it is investigated that following reasons existed, such as a larger vibration in power frequency caused by an unbalance effect on the impeller of the induced draft fan, a larger vibration in frequency doubling and a fluctuation of fan vibration with the change of operation condition caused by the insufficiency and asymmetry of support stiffness. The vibration of induced draft fan with different openings of rotor blade has been finally controlled within a reasonable level through decreasing the amount of unbalance by field dynamic balancing test, adding welding ribs, and increasing the support stiffness with secondary grouting reconstruction.