Abstract: The damage of multiple cracking in the interphase may occur before the fiber breaking,which causes the strength of the fiber to descrease seriously.In this article,with four-phase concentric circular cylinder mode (that is,fiber,interphase,matrix and composite material),and assuming that the interface between the fiber and the interphase can't transfer shear stress after debonding,the stressconcentration of the fiber and the interphase caused by the multiple cracking were calculated by using of shear-lag method,then with Weibull distribution function,the change of the cumulative probability of fiber failure was discussed.Finally,the influence of the interface shear strengthτ₀ on the fiber strength was studied.It is fund that there is the optimum interface shear strength τ₀.