There are lots of factors,such as the refractive index of dielectric,the number of periods,the thickness of defect layer,the width of spectrum line et al.,have effects on the bistable threshold of nonlinear microcavity. However,the red shifting in cavity-mode plays a very important role. We theoretically analyse and numerically simulate the effects of red shifting on the bistable threshold. The red shifting should be first considered in studying bistable threshold.