Estimating additional damping ratio of the damping structure quickly and accurately can improve work efficiency for the design process. In this study, a calculation method of additional damping ratio is proposed based on the conception of effective modal damping energy dissipation, which has an explicit physical meaning. The time varying characteristics of the effective damping ratio are considered in this method. Furthermore, the advantage of this method is its simplicity and high accuracy of calculation. The accuracy of the method proposed in this paper has been proved by a practical case and a comparison between this method and the method proposed in seismic code is conducted.