The effects of strain on electronic structures and optical properties in Si doped anatase TiO₂ (A-TiO₂) were studied by the first-principles calculation based on density functional theory. The results indicate that Si_Ti substitutions cannot improve the optical properties effectively, while Si_O defects can upgrade the absorption amplitude for visible and infrared photons, and cause a redshift in the absorption edge of the optical absorption spectra obviously. The improved optical properties are closely related to electronic structures of Si doped TiO₂ systems. Si_Ti substitutions cannot significantly alter the electronic structure in Si_Ti doped TiO₂, which results in the optical properties of Si_Ti doped system are similar to those of the intrinsic TiO₂ system. However, the impurity levels introduced by Si_O are shallow acceptor levels and locate in the bandgap of TiO₂, which can greatly improve the complex dielectric function in the low-energy region, promoting absorption and photoelectric conversion efficiency of low-energy photons, and therefore enhancing photocatalytic performance of the system. In addition, the optical properties are also related to doping concentration of Si_O, which achieve the best with concentration of 3.7%. On the other hand, the tensile strain of 2% can further increase the absorption amplitude of visible and infrared photons for the system with a doping concentration of 3.7%. Therefore, 3.7% Si dopant and 2% tensile strain in TiO₂ may lead to better photoelectric conversion efficiency and photocatalytic activity. The study may provide a path for improving the optical properties of TiO₂ through both doping Si and strain engineering.