Abstract: Large-scale hydropower station often plays the role of frequency and peak regulation in electric power grid, so the load regulation features a high frequency and a large span. To ensure the stable operation of the generator unit, it is necessary to avoid the vibration area for the Francis turbine generator unit. Because of the system requirements and the unit's characteristics, the governor system (its mechanical components in particular) will undergo damages in varying degrees. The configuration and operation process of a governor's hydraulic control system of a large-scale hydropower station generator unit were introduced. Taking the fracture of the mounting bolt of the main distributing valve pocket as an example, the cause, process and treatment of the fractured mounting bolt as a result of the valve plug jamming were analyzed in terms of production and installation, operation condition, materials of the equipment. The study can provide reference and help for the fault treatment of this type of accident of other similar hydropower stations.