The significant size of blasting blocks in super-large cross-section hard rock caverns will have a profound impact on blasting construction efficiency and process connectivity. Engineering practice has demonstrated that the horizontal structural planes within the rock are among the primary factors contributing to the large blasting block size in hard rock caverns. To investigate the mechanism by which the horizontal structure of a hard rock mass affects the propagation of blasting energy, a hole layout optimization method tailored for complex jointed rock masses is proposed. Using the blasting construction of the middle step in a specific super-large hard rock chamber with a particular cross-section as the engineering background, a three-dimensional blasting model of the rock mass with horizontal joints is established through numerical simulation, based on the RHT model. The LS-DYNA finite element analysis software was employed to elucidate the coupling mechanism between structural planes and blasting energy, as well as the influence of hole distribution patterns on block size distribution. The results indicate that blasting-induced damage and fractures preferentially propagate along the weak surface of the structure. The reflection and superposition of stress waves at the structural surface facilitate the preferential development of radial fractures along vertical or nearvertical structural planes, thereby delaying energy release. The plum blossom-shaped hole distribution ensures more uniform energy distribution through multi-directional stress interference, inhibiting the formation of dominant through-fractures and resulting in a honeycomb-like fracture structure. This effectively reduces the incidence of blasting large blocks and significantly enhances the uniformity of block size distribution. Finally, the validity and feasibility of the conclusion were confirmed through field tests. This study offers a theoretical foundation and practical guidance for controlling the blasting block size in hard rock masses characterized by prominent horizontal structural planes.