Under the condition of large incidence angle in an axial compressor, corner separation will occur in corner region. When the blade loading increases, there may even be corner stall. This three dimensional complex flow structure is high intermittent and unsteady. In order to study the flow mechanisms, a hybrid RANS/LES turbulence model (SBES) was used to simulate the corner separation in the cascade. Firstly, the evolution of vortical structures in the corner separation region were analyzed. The boundary layer and the backflow in the corner separation region encounter, forming vortices with opposite rotation and developing downstream continuously. This cause the unsteady flow in corner separation. In addition, the shedding of detached eddies at the trailing edge is another sources of unsteadiness. Secondly, based on studying the turbulent characteristics, it can be seen that there is active turbulent fluctuation in the corner separation region. The turbulence is high anisotropic because the distribution and the values of Reynolds stresses in different directions are quite different. Thirdly, the mechanism of bimodal phenomenon was explained in this paper. Bimodal means there are two peaks in the velocity probability histogram, it is found in the region between separated flow and non-separated flow. This indicates that there are two non-periodic switching flow modes. Through the analysis of this paper, we find that it is related to the development of large scale coherent vortical structures.