The energy band structures, density of states and optical properties of Ca2Si and P-doped Ca2Si were calculated systemically by density functional theory of the first-principles pseudo potential wave method. The results show that the energy bands of P-doped Ca2Si move to lower energy but it′s still a direct semiconductor with the band gap of 0.557 95 eV. The valance bands of orthorhombic P-doped Ca2Si are mainly composed of 3p of Si, 3p of P and 4s, 3d of Ca. The conduction bands mainly composed of 3d of Ca. Furthermore the static dielectric constant, refractive index, the reflectivity, the absorption and the loss function of orthorhombic of P-doped Ca2Si were analyzed in terms of the calculated band structure and densities of states. The results show that the electronic structure and optical properties of Ca2Si can be effectively modulated by P-doped.