The bulk of Ca3Si4 is investigated by using first principles pseudo-potential calculations based on the density function theory. The results show that Ca3Si4 is a semiconducting material with an indirect band gap and the band gap is 0.372 eV. The valence bands of Ca3Si4 are mainly composed of Si 3p as well as 3s, the conduction bands are mainly composed of Ca 3d. The optical properties of Ca3Si4 are that the static dielectric constant is 19, the refractive index is 4.35, the absorption coefficient maximum is 1.56×105 cm-1, and the value of the loss function becomes maximum at 8.549 eV.