We report a detailed theoretical study of the electronic structure, phase stability, elastic and mechanical properties of Si3B in the pressure range of 0-160 GPa by employing the crystal structure analysis by particle swarm optimization (CALYPSO) method combined with first-principles calculations. Our theoretical predictions reveal that, as the pressure increases, Si3B moves through the following sequence of phases: P3121 ��� C2/m ��� P21/m, and the corresponding transition pressures are computed to be 30 and 64 GPa, respectively. The results of band structures, density of states and electronic localization functions indicate that all three phases act as metallic with strong covalent bonding. The Vickers hardness of C2/m and P21/m phases has been estimated by Gao's, Lyakhov-Oganov's and ��im��nek's models, implying that Si3B is a potential hard material with a hardness value of ���20 GPa. The superconducting critical temperatures of polymeric Si3B are also uncovered to be 3.6 K for the C2/m phase at 50 GPa and 5.7 K for the P21/m phase at 100 GPa. Our results enrich the crystal structures of the Si-B system and provide a further understanding of structures and their properties.