Abstract: A damage evaluation method for bridges, using their online dynamic responses under passing vehicles, is proposed. The perturbation of the elemental flexural stiffness is used as an index for bridge damage identification. The train-induced bridge responses and the response sensitivities to the damage indexes are calculated based on the train-bridge dynamic interaction theory. Using the response differences between different states as restriction conditions, the sensitivity equation is solved by the least-squares method to get the damage indices, and then the damages are located and quantified by the finite element model updating technique and numerical iteration. A numerical example shows that the proposed damage identification method is insensitive to the railway irregularities and the measurement noises, and that the absolute damage or relative damage of the structure can be effectively identified by acceleration, velocity or deflection responses in railway bridges.