To exist inaccurate damage location for current flexibilitybased method for damage localization，a new damage index based on the orthogonal relation between stiffness matrix and flexibility matrix is presented combining with the matrix perturbation method and the finite element method. It can obtain accurate damage location using several order modal parameters. The effectiveness and superiority of this approach is verified by a numerical simulation for the plane truss，and it will be possibly used on damage localization for complex structures.