Abstract: The traditional Kelvin-Helmholtz notion of studying the shearinst ability is not suitable to the shear line with the strong horizontal wind shear.For this case,the shear line should be considered as the vortex sheet.Since then,the instability of this kind of the shear line becomes the instability of the vortex sheet rather than the traditional shear instability.If the velocity is induced by the vortex sheet,the in equalities, (1-Rv+Rid)0 and U(y,t)>U(A(t)) become the criterion of the vortex sheet instability.This criterion indicates that the disposition of environment field rest rains the disturbance developing along the shearline,there exist multi-scale interactions between this mesoscale disturbance and environment field.The calculation of the necessary condition for the instability is also presented in this paper.