Long-period structures (e.g. Isolated structures) tend to produce pseudo-resonance with low frequency compo-nents of long-period ground motions, resulting in the increase in damage. Stiffness mutation occurs due to the set-back in the upper body of the large chassis structure. In the parts with stiffness mutation, the torsion effect caused by the tower is far greater than that of the chassis itself. In this study, a total of 273 ground motions are collected and then filtered into four types, including the near-field ordinary, near-field pulse, far-field ordinary, and far-field harmonic. An 8-degree (0.2 g) fortified large chassis base-isolated structure is established. Furthermore, ETABS program software is used to conduct nonlinear time history analysis on the isolation and seismic model under bi-directional earthquake ground motions. The comparison results show that the seismic isolation effect of the base-isolated structure under long-period ground motions is worse than that associated with ordinary ground motions when the seismic response reduction rate of the large base floor significantly decreases compared with that of the tower. When the inter-story displacement angle and the displacement of isolation layer of the chassis exceeds the limit of Code for Seismic Design of Buildings (GB 50011-2010), it is recommended to adopt composite seismic isolation technology or add limit devices. Under the condition of long-period ground motions, the base-isolated structure reduces the lateral-torsional coupling effect of the large chassis structure, while the torsion response of large chassis\' top layer increases. Under long-period ground motions with the same acceleration peak, the response of the base-isolated structure increases much more than that of the seismic structure and the consideration of this impact is suggested to be added to the Code.