A series of bias extension tests was carried out on balanced plain woven composite preforms with various aspect ratios, disclosing that different aspect ratios may result in different initial failures. An energy method is adopted to quantify several deformation modes. By the competition of the required energies, the initial failure is predicted, showing a good accordance with the experimental observation. The results of the present research are valuable for the further understanding of the material’s behaviour in bias extension test. It also provides an effective way for modelling the material’s formability in other more complicated forming processes. A series of bias extension tests was carried out on balanced plain woven composite preforms with various aspect ratios, disclosing that different aspect ratios may result in different initial failures. An energy method is adopted to quantify several deformation modes. By the competition of the required energies , the initial failure is predicted, showing a good accordance with the experimental observation. The results of the present research are valuable for the further understanding of the material’s behavior in bias extension test. It also provides an effective way for modeling the material’s formability in other more complicated forming processes.