Abstract: Ballistic shielding layers, recognized as a critical component in modern military defense systems, are designed to protect strategically important rear assets. Given the prevalent application of block stones as primary filling materials in these protective structures, comprehensive investigations into their fundamental ballistic resistance mechanisms and practical performance optimization have become increasingly crucial for defense engineering. Therefore, the dynamic fragmentation behavior of block stones under high-velocity projectile impact was systematically simulated using discrete element spherical particles and bonded particle model(BPM) in this paper. The process of rigid projectile vertical penetration into densely packed block stone structure was numerically simulated, with the influences of block stone particle size, shape, and spatial arrangement characteristics on its anti-penetration performance being explored, thereby revealing the underlying mechanism of the shielding effect of block stone structure. The results revealed two primary energy dissipation mechanisms governing the shielding layer's performance: over 90% of the projectile’s kinetic energy was dissipated through the combined effects of inter-block collisions and sliding friction during the penetration process. Meanwhile, the number of break block stones is found to be negatively correlated with the block stone particle size and positively correlated with the aspect ratio (long-to-short axis) of the block stones. Moreover, when single-sized block stones are used in a multi-layer staggered arrangement, the penetration depth of the projectile is primarily determined by the magnitude of the peak penetration resistance. Specifically, the maximum penetration resistance and the minimum penetration depth are exhibited by the scenario with circular block stones of 120mm particle size However, when a layered arrangement with block stone particle size gradiently decreasing along the direction of the impact face is employed, the overall blast-resistant effect of the structure is not effectively enhanced. Finally, the results of the study can provide a reference for the optimal design of the ballistic shielding layer.