Abstract: This paper describes a study on rotor-stator interaction (RSI) and frequencies of flow-induced vibrations in the spiral casing of a high-head Francis turbine unit #1 at the Xiaotiandu hydropower station, using prototype tests and numerical simulations. The tests were aimed at the amplitude and frequency of pressure fluctuations and vibrations, and whole passage simulations of unsteady turbulent flows were focused on an analysis of the RSI between the guide vanes and runner and a detailed examination on pressure fluctuations in the spiral casing and vaneless space. Both the tests and simulations verified that RSI had induced abnormal vibrations in this turbine unit. We analyzed the RSI effects produced by different combinations of runner blade number and guide vane number to solve the problem of vibrations observed during the field tests. Through an analysis on resonance conditions and by applying the principle of optimally matching these two cascade systems to the results obtained from the combinations, we developed an effective method for reducing pressure fluctuations in vaneless space. The results would provide a practically feasible scheme for vibration and noise reduction at hydropower stations.