Disturbance modes and flow structures were numerically analyzed for a two-dimensional spatial evolving supersonic plane free shear layer at Ma_c = 1.2.The disturbance modes and the disturbances parameters of streamwise stations were analyzed with linear stability theory(LST) method and disturbance form direct numerical simulation(DNS) method.The disturbance wave evolving of specially designed disturbance frequency was simulated with disturbance form DNS method.It is found that the multi-mode disturbance without phase difference is produced by the specially designed disturbance frequency in the shear layer.