Abstract Many key military targets have been moved underground to improve the battlefield survivability. To study the influence of explosion distance on the dynamic mechanical response of underground fortification, the full-time numerical simulation is conducted based on the CDEM numerical method. First, four numerical simulations corresponding to different explosion distances are conducted. Then, the displacement characteristic of underground fortification and rock mass is analyzed. Finally, the fracture characteristic of underground fortification and rock mass is analyzed. The numerical results show that the damage degree of underground fortification and rock mass weakens with the increase of explosion distance. When the distance increases from 5 m to 20 m, the volume of rock entering the underground fortification decreases from 24.78 m3 to 0 m3. The time-history curve of crack ratio has the four-stage characteristic, and it increases rapidly during the explosion initiation process. As the explosion distance increases, the top concrete wall mainly undergoes tensile fracture.