The geometric structure, energy band structure, density of electronic state, and optical properties of rare-earth elements La, Y single-doped and co-doped Ca2Si are calculated by the first principles. The calculation results show that, the La single doping and La/Y codoping make the cell volume of Ca2Si increase and the bandgap become narrow, while the Y doping does make the cell volume decrease and the bandgap become wide. After the La, Y single doping and codoping, the Fermi level enters the conduction band, and Ca2Si becomes an n-type semiconductor, and the dielectric function, extinction coefficient, and absorption edge of Ca2Si all move towards the low energy direction, and both the refractivity and reflectivity increase.