To meet the needs of rapid calculation of large amounts of data for structure health monitoring, the structure damage identification method is explored based on strain signals under different input. The theory of damage index method (DIM) based on strain energy distribution change is proposed, and it which is differ ent from the early damage index method based on curvature mode. The values of DIM in different elements are calculated through the comparison of strain energy in different elements, when the strain response of structure both in healthy and in damage is abstained. Based on the Matlab and ANSYS numerical simulation, an example of long-span cable stayed bridge is analyzed. The results show that the damage position is accurately located under different incentives and the capacity of anti-noise pollution is improved.