Abstract: Ice accretion on a wing can seriously affect the flight safety. Analysis on the unsteady flow characteristic of the iced wing, especially of the horn ice near the stall point, is a vexing challenge which receive significant research interest in recent years. In this study, unsteady and complex flow separation around a GLC305 airfoil with 944 horn ice is numerically investigated at first. Besides, the proper orthogonal decomposition (POD) and the dynamic mode decomposition (DMD) are introduced to extract modal features from the turbulent wake behind the three-dimensional wing with ice shape. The results revealed that the IDDES method accurately predicts the lift coefficient and pressure platform behind the horn ice, and clearly captures the vortex structures and its evolution process. The first two modes decomposed by the POD method occupy the main energy, and the frequency corresponding to the time coefficients of the first four modes is consistent with the shear layer oscillation frequency in the experiment. Besides, the dominant frequencies and the corresponding magnifications are extracted with the DMD method. It is found that several highly unstable modes lead to flow separation, because these modes correspond to flow structures that are uncontrolled growth with time. With both the POD and DMD methods, it can be noted that the energy sequences of their main modes occur start from the middle position of the airfoil, which is related to the position where the shear layer loses its stability.