Kelly and Xiyuan Zhou research on upper single-degree-of–freedom model of regular isolated building, and give necessary computing formulas. For various middle or low isolated building, the result is satisfied by using the model. Recently isolation technique was applied to high-rise buildings. For isolated building systems with large aspect ratio, high-order mode parts of upper structure can not be ignored. It brings large errors to use single-degree-of–freedom model. In the paper, according to the equivalent rules of equal structure period and total base shear, the computing formula of structure parameters is deduced. After numerical analysis for some regular buildings, a simplified method for equivalent model is given. By analyzing the earthquake responses of equivalent model and original structure, it is proved that the equivalent method is simple and feasible.