By the finite element methods (FEM),the global coupling dynamic model of the main shaft system of a Francis turbinegenerator unit is established, which includes electromagnetic parameters, structural parameters and fluid parameters .Based on the existence of transverse vibration and torsional vibration of the turbine shaft system, the hydraulic excitation exerting on the main shaft system, which is translated by the blades, is derived. According to the dynamic equations of the main shaft system, the internal relations between the hydraulic excitation and the vibration characteristics of the main shaft system are discovered. The studies show that the hydraulic excitation can be transfered by the turbine blades as middle transform components to exert on the main shaft system, and that the hydraulic excitation is not only related to the complicated pressure fluctuation in turbine runner, but also to the structural parameters and the vibration of the main shaft system, and that the hydraulic excitation and the vibration of the main shaft affect with each other. The fluidsolid coupling vibration of the main shaft system of the Francis turbine generator unit may take place subjected to the hydraulic excitation. An example is presented at last.
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