Abstract: Long period far-field quasi harmonic earthquake motion easily leads to strong seismic response of isolated structures. The isolated structure designed based on the rigid foundation assumption has a good damping effect in the actual soil layer, but the soil structure interaction（SSI effect） cannot reflect a good damping effect. Therefore, based on the shaking table test results of the existing isolated structures with large chassis and single tower on the rigid foundation, soft soil foundation and soft interlayer foundation, a comparative analysis is carried out to discuss the differences in dynamic response and seismic reduction performance of the three types of isolated structures under far-field harmonic ground motions. The results are as follows. Under the SSI effect, the natural period extension times of non-isolated and isolated structures on soft soil foundation are slightly larger than those on soft interlayer foundation. Under different types of ground motions, the distribution laws of acceleration amplification coefficients of isolated structures on soft soil and soft interlayer foundation are similar, and the acceleration responses of upper and lower structures of interlayer isolated structures are different after considering SSI effect. The displacement response of isolated structures on soft interlayer foundation under far-field quasi harmonic ground motions increases more significantly than that on soft soil foundation; The angular acceleration of the base rotation of the upper layer isolation structure on soft soil foundation is slightly greater than that on the soft interlayer foundation.