Abstract: In the process of structural damage detection, the incompleteness of experiment mode shape is the main obstacle which hinders the efficiency of applying structural damage detection technique. A damage detection method is presented here which solved the qualitative and quantitative identification based on the reduced modal strain energy and frequency. The finite element model is reduced by Neumann series expansion. The reduced modal strain energy is defined, which is proved sensitive to damage. The variation rate of reduced modal strain energy is taken as the damage signature to identify the location of damage. With the probable damage locations determined, the eigenvalue perturbation method was employed to estimate the damage extent. The proposed method is demonstrated through the example of a simple steel beam. The results show the present method could give good damage detection results even if noise is present in test modal data.