Buoyant model experiment and numerical simulation of flow field were performed to investigate the mechanism of flow induced underwater cavity and the problem of frequency prediction. Normal fairwater model and opening fairwater were chosen in the experiment. A series of vibration line spectrums caused by the opening were detected. Detailed flow field of the opening model was established and simulated with large eddy simulation. The pressure oscillation frequencies of simulation agreed well with the vibrating acceleration line spectrums, which indicated that the line spectrums detected in the buoyant model experiment were excited by the self-sustained oscillation of the cavity. Observations of the vorticity and fluctuating pressure around the opening prove that the self-sustained oscillation of the underwater cavity was caused by the mechanism of the feedback of hydrodynamics static pressure. Analysis of the frequencies of self-sustained oscillation shows that the classical Rossiter formula is not applicable in the underwater situation, which should be modified based on the buoyant experiment data.