Abstract: Double-mass simplified calculation model, with the consideration of higher mode effects, has been established as a practical calculation model in simplified calculation research of base-isolated structures. Sensitivities of seismic response and dynamic characteristics to mass ratio and period ratio of the double-mass simplified calculation model were calculated and illustrated based on two fundamental conditions: mechanical properties of base-isolated structures and the sum of mode-participation vectors of each mass point being 1. Furthermore, the application range of the simplified calculation model and relevant formulas were derived and analyzed. An eight-story base-isolated structure was taken as one numerical example to verify results of theoretical derivation involved. Results show that the sensitivity of seismic response to period ratio and mass ratio is not more significant than that of dynamic characteristics; and the secondary modal damping ratio, obtained from formulas proposed by Kelly and later improved by scholars such as Li Zhongxi, may be seriously apart from the suitable period ratio and mass ratio of simplified calculation model.