Abstract: Numerical simulation was employed for the qualitative research on unsteady vortex shedding frequency in two-dimensional subsonic compressor cascade. Fast Fourier Transform Algorithm was adopted for the value of multi-dots' total pressure in nine different flow conditions. An acceptable criterion is advanced to distinguish the physical oscillation from the numerical oscillation; the vortex shedding frequency in numerical simulation is obtained under the nine different unsteady separated-flow conditions. In non-designed situation, large-dimensional unsteady open separated-flow emerges on the suction surface in two-dimensional subsonic compressor cascade and periodicity vortex shedding might be the primary movement fashion in separated-flow area and wake area dominated by separated-flow.