The helix geometry model of 3D braided composites has been presented, which truly reflects the braided manner and coincides with the actual configuration of the braided composites. The longitudinal tensile stress-strain relationships and the strength of 3D braided composites under the tension loading have been predicted by a finite multiphase element method (FMEM) based on the helix geometry model. Comparisons are conducted for those from the present model and experiment. The results obtained from the present model are supported by the experimental data. The numerical results show that the braiding angle has a significant influence on the strength of 3D braided composites.