Based on theoretical derivation and numerical simulation methods, a systematic study was carried out on the seismic stability of a series isolation system after isolation retrofitting, which is composed of laminated rubber isolation bearings and the lower pillar of the basement. The laminated rubber bearing was simplified to a special hinge bearing with horizontal stiffness and flexural rigidity, while the RC column was simplified to a curved vertical rod. Then a theoretical model of the series isolation system was established, and the equation for the critical bearing capacity of the lower pillar of the basement was derived. The concrete expression of the critical bearing capacity was solved based on practical cases, and the influences of typical parameters were analyzed. Six kinds of column top isolation models with different lower pillar section dimensions were established by the numerical simulation method. Then the overall displacement response of the system and the response of the story drift ratio in the superstructure were compared and analyzed. Results show that the variation of deduced expression for the critical bearing capacity was consistent with the numerical simulation results of the column top isolation model. The increase in the cross section of the lower pillar has no obvious effect on the dynamic response of the superstructure and the isolation bearing, but it could remarkably reduce the displacement of the lower pillar. In the actual seismic isolation reconstruction project for existing buildings, increasing the lower pillar section size is a simple and effective method for ensuring that the seismic capacity of the lower structure is higher than that of the upper structure. However, in practical projects, the section increment is generally large and conservative, resulting in some waste.