Abstract: In this paper, according to the stiffness distribution characteristics of base isolation building system, the perturbation method for dynamic analyses of such system is developed, in which the ratio of the upper structure stiffness to that of the lower structure is adopted as perturbation parameters. Theoretical derivation and numerical examples show that the method can clearly demonstrate the influence on the dynamic responses of the structure by various orders of differential dynamic forces. As the variables are the same in each calculation level, so the rationality, reliability and stability of calculation results could be ensured. The proposed method can be applied to complex base-isolation system for dynamic analysis.