Abstract: The vortex formation and evolution in a supersonic air/air free diffused plane mixing layer were investigated numerically by forcing four different harmonic disturbances without phase differences.The two-dimensional compressible Navier-Stokes equations including the diffusion effect of air components were solved by using a high accuracy finite difference method.The convective terms and transport terms were discretized by a third-order upwind compact scheme and a sixth-order symmetric compact scheme,respectively.The unsteady time marching method is a third-order compact storage explicit Runge-Kutta algorithm.Some phenomena of large-scale primary vortex growing and interacting were revealed,such as saturating,paring once and twice,and triple-vortex pairing.For low convective Mach number of Ma_c=0.3,primary vortices are fatter,but their streamwise lengths are shorter.Under the influence of air real-gas characteristics,the vortex-shocklet structure is not discovered in the case of high convective Mach number of Ma_c=0.8.