Abstract: A homogenization method based on the perturbation theory is presented to deduce the mathematical expression of the micro-stress. The FEM method was used to simulate the micro-stress of three-dimensional braided composites. The proper criteria of strength were used to judge the failure of each element under the loading of the limited tensile stress. A kind of micro failure criterion of strength can be concluded. The calculation results of stress obtained by this method agree quite well with some conclusions of the tensile experiment.