Abstract: The outer sleeve-conductor system of ultra-high voltage (UHV) bushing has the risk of electrical function failure under seismic action. In order to study the internal insulation failure mode of UHV bushing caused by the excessive displacement, a simplified theoretical model of an outer sleeve-conductor coupled system is established, and its analysis results are verified by a ±1100 kV UHV gas-insulated switchgear (GIS) composite bushing shaking table test and by the finite element method. The interaction mechanism and seismic vulnerability of the outer sleeve-conductor system are studied. The study results show that the main failure modes of internal insulation under seismic action are caused by two aspects: a) the deformation of the conductor caused by displacement at top of the outer sleeve; b) the seismic excitation of the conductor itself. And the former is the main cause of internal insulation failure. The relative displacement of the outer sleeve and the conductor shows an approximate parabolic distribution along the height, and the maximum point is located at the lower part of the bushing gravity center. Under the actions of the same PGA, the failure probability of internal insulation is between the failure probability of fracture at the bottom and the traction at the top of the bushing. The seismic response of relative displacement between the outer sleeve and conductor can be obtained by the simplified theoretical model, and attention should be paid to both internal insulation and fracture failure modes.