In order to investigate the reciprocity between large-scale structures and pressure oscillation for better understanding the process of mixing deeply, LES simulation was used to investigate the evolution of 2D supersonic planar mixing layer in this paper. The numerical results show the merging process of vortices conforms well with the relative experimental results. Through observing the Fourier transform of the pressure at a spatial point, the results show that under the hypothesis of periodic motion of vortexes, at any time, the frequency of large vortex that behind the crunch is just the peak value of the frequency of the pressure.