We discovered the composition of ternary ABSi (A = Na/K/Rb/Cs) compounds in the moderate pressure range of 0-100 GPa using first-principles structural prediction and systematically analyzed their structures, stability, electronic and optical properties within the framework of density functional theory. The ABSi compounds exhibit a diverse phase diagram, including nine structures that are selected based on formation energies, along with a known clathrate RbBSi structure with Pm3̄n symmetry. All predicted phases are thermodynamically and dynamically stable within the studied pressure range. In particular, the KBSi compound with a direct band gap of 1.0 eV is identified as a promising candidate for photovoltaic materials beyond silicon-based materials, among which boron atoms form a unique regular octahedral structure; in contrast, NaBSi and RbBSi compounds are shown to have metallicity. Our findings enrich crystal structures of alkali-metal borosilicides and provide valuable insights into their potential applications.