Abstract: The temporal spatial discrete Fourier transformation is employed to analyze the dissipation and dispersion errors of the numerical algorithm used here. Based on directly solving the two dimensional time dependent compressible Navier Stokes equations, the spatial evolution of primary vortex generated in forced compressible plane free shear layers is simulated numerically by using a new fifth order upwind compact difference scheme. The phenomena, including the primary vortex saturating, the first pairing, and the second pairing, are shown with the passively conserved scalar method and so on. The initial value effect related to subharmonic disturbances in incoming flows is studied. The results show that the spatial evolution type of large scale compressible vortices is associated with the disturbance modes.