Abstract: The damage in a steel truss model bridge is simulated by changing its element stiffness. The dynamic behaviors of the model bridge are analyzedwithFEMdynamic structural analysis program before and after the model bridge is damaged. It is found that the changes in the mode of vibration are sensitive to the damage degree of elements, and there exists a corresponding relationship between them. A new method for weighting the changes in modes during the structural damage is proposed, inwhich the modes of structural vibration are chosen as weight coefficients. Element damages can be oriented effectivelywith this method.