Based on the first-principles calculations, the strain effects on the optical properties of direct band gap silicon crystal D135-Si were systemically investigated. The stress-strain relations and phonon spectra show that the realistic peak tensile strengths in three principle symmetry crystallographic directions [100], [010] and [001] are 4.5, 9.7 and 12.1 GPa, respectively. By imposing the pressure and strain on structure, we studied the direct-indirect band gap transitions and obtained the ranges of the direct band gap, namely 0-4 GPa on pressure, -0.07-0, -0.04-0.08, and -0.01-0.03 on strain along a-, b- and c-direction, respectively. The imaginary part of the dielectric function was calculated to analyze the optical absorption property, which shows the strong adsorption coefficients in the visible range of the sunlight. The effect of pressure on the optical absorption property of structure is the smallest, and the strain effects are gradually enhanced along c-, b- and a-direction, respectively.