Abstract: A 2.5D finite element-boundary element (FE-BE) coupling method is proposed for the 3D seismic responses of an underground tunnel subjected to obliquely incident seismic waves. The tunnel linings are discretized with finite elements and the effects of the segment joints are simulated by spring elements, while the unbounded soil is modelled by the boundary element method based on the 3D dynamic stiffness matrix of a layered half-space and the Green's functions for moving distributed loads. The proposed 2.5D FE-BE coupling method is of high computation efficiency and accuracy, and its correctness is verified by comparing with the benchmark results in the literature. The 2.5D FE-BE coupling method is used to study the seismic responses of a shield tunnel, with an emphasis on the effects of the segment joints on the deformations and internal forces of the tunnel linings. Moreover, the applicability of the equivalent homogeneous lining model, which approximately considers the effects of segment joints by reducing the bending rigidity of the tunnel linings, is examined. It is shown that compared with a homogeneous tunnel, the shield tunnel can accommodate larger deformations while generate smaller internal forces. Furthermore, for the case of joints with large stiffness, the equivalent homogeneous model for linings can account for the effects of the segment joints to a certain degree, but it may lead to underestimation of the longitudinal bending moment of the tunnel linings.