Abstract: It is an economical and effective method to enhance the aseismatic capability of buildings by using passive energy dissipation structures. But in the analysis of this type of structure with mode-superposition response spectrum method based on equivalent linearization, it is rather difficult and time-consuming to obtain satisfactory results. An accurate expansion method for modes based on the improved first-order perturbation is presented in this paper. A relatively uncoupled mode-superposition method and a modified first-order perturbation method are derived. A numerical example shows that the present strategy is correct and effective.