Abstract The mechanical responses of underground fortifications subjected to blast loading is an important topic related to survival and security ability. Based on a FEM-DEM coupled algorithm, the difference in mechanical responses of underground fortifications subjected to different blast loadings is studied. Firstly, full-time numerical simulations of underground fortifications are conducted. Then, the mechanical characteristics (i.e., displacement and crack) of underground fortifications are analyzed quantitatively. The result indicates that the maximum displacement of underground fortifications gradually reduces from 3.35 m to 1.90 m when the horizontal distance D H increases, and the rock volume V C and V F gradually decrease with the increase of D H. The crack ratio at the same moment gradually decreases with the increase of D H, and the spatial distribution of fracture type changes significantly with the increase of D H.