A sandy soil liquefaction interlayer can cause both foundation failure and damping effects on the superstructure, while the overburden would reduce building settlement but increase seismic effects. Therefore, it is necessary to consider the seismic coupling effects of the liquefied interlayer and the overburden on the building under the structure-soil interaction. Based on the Biot two-phase saturated porous medium dynamic coupling theory, the effects of seismic intensity, sand compactness D_r and liquefied layer thickness-to-depth ratio K (considering the coupling effect of liquefied interlayer and overburden) on the seismic isolation of the superstructure are analysed using finite element and finite difference coupled dynamic analysis methods. The results show that the structural acceleration spectrum ratio first decreases and then stabilises with increasing seismic intensity, and the damping effect of the liquefied layer no longer increases when the seismic intensity exceeds a certain threshold; the damping effect of loose sand (D_r=30%) is better than that of medium-density sand (D_r=50%) and dense sand (D_r=70%), but it would cause a non-negligible settlement hazard, while dense sand, although it will better suppress the settlement amount of the building, would lead to amplification of vibration; considering the coupling effect of overburden and liquefied layer, it is found that there is a K-value interval (0.3<K<0.5), which can both reduce building settlement and act as shock absorber.