Earthquake-induced liquefaction can lead to uplift displacements of underground structures. While the behaviour of a single underground structure has been extensively studied, the coupled response of a shallow buried circular tunnel adjacent to an underpass with a rectangular section in liquefiable ground remains unclear. In this study, the finite difference method based on the fully coupled effective stress is employed to analyse the seismic response of two adjacent underground structures. Firstly, comprehensive parameters of Hostun sand for P2Psand constitutive model was calibrated. The accuracy of the numerical method is verified through a centrifuge test. Subsequently, comparisons are made between the seismic response of the tunnel-soil-underpass interaction and other cases: free field, tunnel, and underpass. The tunnel-soil-underpass interaction in liquefiable ground is analysed, with a special emphasis on the development of excess pore pressure, acceleration response of the soil, displacement and deformation of the structures. A section deformation coefficient was proposed to depict the tunnel deformation. Finally, horizontal spacing, buried depth of tunnel and underpass are discussed to clarify the interaction of two structures. The optimal relative position of two structures is suggested, providing valuable insights for the spatial design of underground structures in liquefiable ground.