Abstract: The complex separation flow field over a modern fighter model was simulated by using an unstructured/hybrid grid based detached eddy simulation (DES) solver with an adaptive dissipation second-order hybrid scheme in this paper. The comparison with experimental data which is limited to averaged aerodynamic coefficients shows that the simulation results are reasonable, then the unsteady properties of DES flow field were investigated in detail by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods. The study indicates that the flow field in leeward side is dominated by a pair of strake vortexes with spiral motions, before the vortex breakdown taking place the flow is neutrally stable in cross-section plane and the main vortex motions contain multiple frequencies. The comparison of POD and DMD shows that though the modes of the two methods can be made pairs in different ways, there are some relevance between the main modes of the two methods. It also can be found that multiple frequencies are contained in each POD modes and mode energies are more concentrated in the main POD modes, so the flow field reconstruction with POD modes is more efficient. While the DMD method resolves the main properties of flow field into a series of modes which are very close to harmonic motions and the stability analysis of these modes can be given.