To study blast loading on the inner wall after the explosion in reinforced concrete box structure, LS-DYNA was used to simulate the internal explosion, the simulated overpressure time history of test points and characteristic points were gained, results were compared with experimental ones. Multiple reflection and superposition between inner walls are the salient features of internal explosion, when charge detonates in the center of rectangular (L: B: H = 2:1:1) one, The points on the inner wall which suffer the maximum overpressure are the nearest points from the explosive center, followed by the vertexes of cross-section across charge center which parallels with the two ends; For the cube one, the points of maximum overpressure on the inner wall are the cavity vertex points, followed by the points right face the explosion center. Areas of intersection of two planes and intersection of three planes will make the clustering effect stronger, positive time longer, and increase the specific impulse.