Abstract: In order to clarify the mechanism of the unsteady cavitation flow, the characteristics of sheet and cloud cavitating flow structure around a hydrofoil are investigated by joint numerical and experimental methods. Experiments on a hydrofoil are carried out in a rectangular test section of cavitation tunnel, a high-speed video camera is used to visualize the cavitating flows. The computations are based on the homogeneous model, the liquid/vapor mixture density is regulated by mass transfer equation. The FBM turbulence model with modified eddy viscosity coefficient is used for the computations with software. A good agreement is obtained between experimental data and numerical simulations. In sheet cavitation regime, it has a quasi-steady characteristic with most of the unsteadiness localized in the closure region. In cloud cavitation regime, it has a cavity generation-development-shedding-collapsing quasi-periodic characteristic, and it can capture the U-shaped cloud cavity and different intensity of the re-entrant jet in spanwise direction towards the mid-span region when the large-scale cavity is shedding, the cavities breaks off from the mid-span region near the leading edge of the hydrofoil and become the vortical cloud cavities, which have the large-scale U-shaped structure.