The response of underground structures subjected to subsurface blast is an important topic in protective engineering. Due to various constraints, pertinent experimental data are extremely scarce. Adequately detailed numerical simulation thus becomes a desirable alternative. However, the physical processes involved in the explosion and blast wave propagation are very complex, hence a realistic and detailed reproduction of the phenomena would require sophisticated numerical models for the loading and material responses. In this paper, a fully coupled numerical model is used to simulate the response of a buried concrete structure under subsurface blast, with emphasis on the comparative performance of 2D and 3D modeling schemes. The explosive charge, soil medium and the RC structure are all incorporated in a single model system. The SPH (smooth particle hydrodynamics) technique is employed to model the explosive charge and the close-in zones where large deformation takes place, while the normal FEM is used to model the remaining soil region and the buried structure. Results show that the 2D model can provide reasonably accurate results concerning the crater size, blast loading on the structure, and the critical response in the front wall. The response in the remaining part of the structure shows noticeable differences between the 2D and 3D models. Based on the simulation results, the characteristics of the in-structure shock environment are also discussed in terms of the shock response spectra.