Abstract: Fibers are all with many types of defects,and tensile strength of single fiber is always determined collectively by these defects.Tensile strength distribution of single fibers which have many types of defects was deduced and proved theoretically not submitting to tow-parameter Weibull distribution,but to poly-Weibull compound distribution.A strength prediction mode of fiber bundles were inferred farther.Aiming at poly-Weibull compound distribution of single fibers strength,numerical simulation results show that the bigger the discrete coefficient of single fiber tensile strength,the more predominant the influence of fiber length on average strength of single fibers and the lower the utilization of fiber tensile strength in fiber bundles.