Abstract: A phenomenological model of collective interaction of primary vortex of mixing layers is built on the linear stability theory. The model can be used to determine the influence of the phase shift of the subharmonic disturbance wave on the nonlinear evolving process of the primary vortex. The results indicate that there are three classes of subharmonic interaction to the short subharmonic wavelength, which are tearing, symmetric collective interaction and asymmetrical collective interaction, respectively. And to the long subharmonic wavelength, there always occurs asymmetrical collective interaction.