Abstract: For the No. 5 boiler flue gas desulfurization system of a coal-fired power plant, to solve the problem of the frequent shafting fault of the booster fan after the operation mode changing from the fixed frequency operation to the variable frequency operation, by comparing the working conditions before and after the frequency conversion retrofit of the booster fan, and combining the change of the fatigue safety factor of the booster fan shafting, it is found that the shafting fault is mainly caused by the increased alternating stress and inertial force during the variable frequency operation, which leads to the fatigue safety factor of the booster fan shafting is lower than the design value under the variable frequency operation. Through the reinforcing retrofit for the impeller, the long and short shaft, the coupling and other parts of the shafting with checking, the existed fault can be effectively solved and the reliability of the equipment can be improved.