Abstract: The NACA0012 airfoil at high angle of attack is simulated by the hybrid SST-DDES method based on the shear stress transport (SST) turbulence mode. The stability of the flow field is analyzed by using the mathematical tool dynamic mode decomposition (DMD) in the initial stalled state, shallow stalled state, and deeply stalled state. The results show that the DMD method can be used to accurately extract the dominant frequency, high order frequency doubling, and the corresponding flow field mode structures in the airfoil flow at high angle of attack. Compared with the fast Fourier transform (FFT) analysis results, the maximal difference of the frequency is less than 0.16%, and the leading role order of both extracted frequencies in the flow is the same. By analyzing the stability of the corresponding mode through eigenvalue, the growth rates of all modes are very small. All modes are weak divergent, weak convergent or in the stable limit cycle state. The first order mode reflects the main static separation vortex structure in the vortex evolution process. The mode vortex corresponding to the first three low frequencies is consistent with the vortex structure evolved in this frequency. The higher order frequencies mainly reflect the trailing vortex and the vortex structure in the wake region. It is found that there are phase differences between the different mode coefficients. This indicates that the movement of the vortex structures of different frequencies is asynchronous in the separated flow.