Underground structures in liquefiable ground are prone to severe earthquake damage, and the seismic responses of such underground structures can be affected by nearby superstructures. Two centrifuge shaking table tests on a precast underground structure with and without a superstructure above it in mildly inclined liquefiable ground were designed and conducted in this study. The seismic response of the underground structure was analyzed in terms of its acceleration, deformation, internal force, and waterproof performance. The influence of the superstructure on the underground structure seismic response, including the deformations and internal forces, was investigated by comparing the responses of the underground structure in the two tests. It was observed that the joints of the precast underground structure experienced greater deformations than the nonjoint positions and affected the overall seismic response of the underground structure. Among the nonjoint positions, the column and corners of the underground structure experienced relatively large deformations and internal forces. The joint-opening deformations and dynamic bending moments of the precast underground structure were mainly governed by soil shear deformation, and the increased soil lateral forces caused by excess pore pressure (EPP) accumulation increased the bending moment of the side walls. The superstructure could amplify the acceleration of the near-field soil and underground structure, thus intensifying the EPP accumulation and underground structure deformations and internal forces. The detrimental influence of superstructures on underground structures in liquefiable ground merits more attention in seismic design.