The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper.The flow patterns and structures in different cavitation regime,for the attached cavitation and the vortical cavitation,are shown with both the visualization and the quantitative information.The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered.In particular,the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation,and the cavitating vortex street\'s forming and crumbling are described.The evolution of the cavitation structure in the transient sheet/cloud cavity forming,along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed.The perspective future research of higher fidelity simulations,and the accurate identifications of the cavitating vortex structure is commented.