By decoupling the nonlinear fluid equations of ion-temperature-gradient (ITG) mode, the zonal flow-drift wave nonlinear dynamical equation including magnetic shear is derived. The role of magnetic shear for zonal flow generation by ITG mode turbulence is studied using a four-wave interaction model of modulational instability. Finally we can draw the conclusion that within a smaller range of k//, as |k//| increases, the growth rate of zonal flow is also increased.