Abstract: The numerical calculation of the cavity flow using 2D-LES method has been conducted to examine how the momentum loss thickness and inflow velocity at the cavity leading edge affect the cavity flow oscillations, and how the length-to-depth ratio of the cavity is related to the oscillation modes of the cavity flow. PIV measurements of cavity flows verify the credibility of the results of numerical calculation. The results show that the friction velocity determines whether or not the flow of water over the cavity will oscillate. The instantaneous vortex structures and the cavity drag coefficients identify two typical modes of oscillation, the shear layer mode and wake mode. The key factor that determines the oscillation mode is the length-to-depth ratio of the cavity.