Abstract: The definition of element damage identification parameters in finite element method (FEM) is firstly presented. The 1st and 2nd perturbation equations for structural vibration eigenvalue problems are derived by combination of perturbation and vibration theories. The related optimization algorithms of these equations (in un-determined situation) for solving damage identification parameters are also proposed. Structural damage location and severity as well as structural aging can be detected using the proposed method with measured frequencies. The proposed method avoids the errors caused by low measurement accuracy or insufficient degrees of freedom when using traditional mode identifying method. Numerical simulation showed the validity of the presented approach. It can be utilized in health monitoring and damage detecting procedures.