Abstract: Cavitation is a special flow phenomenon with strong unsteadiness that often occurs in propeller of ships and underwater vehicles. The occurrence of cavitation often affects the hydrodynamic performance and efficiency of propulsion systems. In order to study the unsteady cavitation flow field structure around hydrofoil, numerical prediction and flow field structure analysis of unsteady cavitation flow around two-dimensional hydrofoil are investigated by using Schnerr-Sauer cavitation model and SST k-\omega turbulence model. The validity of the established numerical method is verified by comparing the numerical prediction of cavitation evolution and pressure data with experimental results. The velocity field of the cavitation flow field is analyzed by using Dynamic Mode Decomposition (DMD). The results show that the first-order mode is 0Hz, which represents the average flow field. The second-order mode is about the frequency of cavitation shedding, which reveals the cavities grow and shed periodically at the leading edge of the hydrofoil. The third-order mode has a corresponding frequency about 2 times of the second order mode, which reveals that the fusion behavior of two large-scale vortices behind the hydrofoil. The fourth-order mode has a corresponding frequency about 3 times of the second order mode, which characterizes the fusion behavior of some small-scale eddies in the flow field. Finally, the modal decomposition analysis of the cavitation flow field under different cavitation numbers was carried out. It was found that the vortex structure of the shedding cavities increased with the decrease of the cavitation number, and the second-order mode frequency decreases with decreasing cavitation number.