First-principles calculations based on density functional theory are performed for zinc-blende B_xGa_1-xAs alloy with a special quasirandom structure (SQS) approach in a 64-atom supercell. Structural,electronic and optical properties of B_xGa_1-xAs alloy are comprehensively investigated under optimized geometry structures. It was found that calculated lattice constant slightly deviated from Vegard's law and significant bond length relaxation is shown. Calculated bowing parameters of energy gap are small (2.57 eV-5.01 eV) and are weakly composition-dependent. Finally,effects of boron composition on optical parameters including dielectric function, reflectivity,refractive index,absorption coefficient and energy loss function are analyzed in detail.