In order to provide reference for loading carrying capacity evaluation and maintenance strategy, the damage detection is performed for long span cable-stayed bridge. Based on the structure feature of cable-stayed bridge, the main girder is considered as a beam on elastic foundation, furthermore, the internal force redistribution only occurs on the elements near the damage. Therefore, the position of damage can be detected according to the change of cables forces in the same load case. Based on the reference FE model, the optimization method is employed. Taking the degree of the damage elements as the design variables and the difference of cable forces between the measured value and calculated value, the FE model is updated in order to identify the degree of the damage elements. Because of the linear relationship between the internal force redistribution and the initial structure deformation, the changes of cable forces under the same truck load are sensitive to the structure damage. Finally, taking the Sutong Bridge as an example, four damage cases are analyzed and identified. The calculation results show that using the reference FE model and the changes of cable forces under the same truck load, the position and the degree of the girder damage can beidentified.