The influence of Mn doping on the Ca2SiO4:Eu2+ phosphors is investigated by the First principle calculation. Comparing with the Ca2SiO4:Eu2+ phosphor, it is found that red-shift of the absorption spectrum for the Mn-doped Ca2SiO4:Eu2+ phosphor takes place. Mn2+ ions substitution on Ca2+ sites causes an increase of the crystal ?eld strength, which results in Eu5d level moving down ward in energy. Besides, Mn3d partly enters into the top of the conduction band and the bottom of the valence band. These two reasons bring about narrow band gap and red shift of absorption spectrum.