Abstract It is of great theoretical significance and engineering application to investigate the influence of detonation interval on the dynamic response of underground fortification under blasting loading. First, a numerical simulation of underground fortification under blast loading is carried out based on the continuum-discontinuum element method. Then, the displacement characteristic of underground fortification is analyzed. Finally, the fracture characteristic of underground fortification is analyzed. The results indicate that the increasing trend of displacement at the top boundary gradually decays with the increase of detonation interval. The final value of displacement at the left boundary first increases and then decreases with the increase of detonation interval. With the increase in detonation interval, the proportion of concrete undergoing shear fracture at the top, left, and right boundary gradually decreases.