Abstract: Severe vibration and noise of bulb turbines at hydropower stations is the main cause of unstable operation. Up to the present, the previous research of bulb turbine stability has focused on rotational frequency and vortex frequency rather than the harm of high power rotational frequency pressure pulsations and their causes. In this paper, the theory of destructive effect of cavitation on hydraulic mechanical stability is applied to study clearance cavitation in bulb turbines, and the relationship between clearance cavitation and such pressure pulsations is demonstrated and analyzed by combining theoretical analysis with prototype and model test verification. This paper describes the pressure pulsations in the draft tubes of model and prototype bulb turbines, along with the vibration frequency characteristics and working conditions in prototype; and points out that the cause for the high power rotational frequency pressure pulsations is the occurrence of cavitation at runner blade tip clearances and guide vane tip clearances. This study proves that the blades passing frequency pressure pulsations and their second and third harmonics are induced by the clearance cavitation between the blade tips and the discharge ring or runner hub, and the guide vanes passing frequency pressure pulsations induced by the guide vane tip clearance cavitation.