Micro-damage is always unavoidably incurred in structure in fabrication and serving performance. It is of high value to develop an undamaged detection method for these structures. Based on the multi-DOF vibration of mechanics and continuum damage theory, strain-mode based undamaged detection method was discussed and applied in the undamaged detection of composite beam. The strain response of the damaged laminated composite beam at different tested points was obtained through the dynamic test of the structures. And the strain-model curves of the damage structures was acquired by dealing with the dates with the MATLAB programs. The result indicates the exact position of the damage in the structure, which proves the validity of the strain-model method on the damage detection of the laminated composite structure. At last, the model analysis was conducted by applying continuum damage theory based on finite element software ABAQUS, which further demonstrates the feasibility and reliability of the strain-mode method in predicting the damage of structure.