In order to explore the causes of the butterfly valve rib plate cracking of a turbine at a hydropower station, we calculated and anlayzed the strength and the vibration properties of the butterfly valve using the finite element software ANSYS. The strength analysis results show that the stress level of a 40-mm rib plate is very low. Obviously, high stress is not the reason for rib plate cracking. The results of modeling analysis show that the Karman vortices occurred at a 40-mm rib plate and a 90-mm cover plate. Referring to the results of FEM analysis, we conducted a site test for the butterfly valve at the hydropower station. The experimental results agreed with the calculated results. Hence, the cracks in the butterfly valve were caused by the Karman vortices.