A simplified formulation is proposed in this paper for the lateral deformation characteristics of high-rise shear-wall isolated buildings. While uniaxial analysis, the building structure is modeled by lumped mass method and the first two modes can be analyzed in the response spectrum method based on the equivalence of modal masses. The periods, modal shapes and damping ratios of the first two modes can be calculated using simplified expressions of the linear stiffness-gravity ratio with the considerations of the rotation of the base isolation. Numerical simulations show the simplified method can catch the dynamic characteristics of the first two modes. The formulations for calculation of the periods for high-rise buildings and buildings with base isolations are given explicitly, respectively which present the theoretical supports for the practical designs of such structures.