Abstract: The flow separation remarkbaly undermines wind turbines' aerodynamic performance thus requires. In order to develop flow control strategies, fundelmental research on the unsteady flow separation is abosutely necessary, and the Reduced-Order Model (ROM) methods offers a tool. In this paper, the unsteady flow fields around S809, a typical horizontal axis wind turbine airfoil, at deep and shallow stall angles of attack are obtained numerically and further analyzed by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Results show that POD and DMD can accurately capture the unsteady flow structures and dominant modes, but the former, based on magnitude of energy, ignores the flow structure with similar lift main frequency but less energy. DMD can accurately capture the flow field evolution (rate of increase, frequency, etc.) and develop targeted flow control strategies for the dominant frequency structure, offering the airfoil flow field conditions and aerodynamic performance.