The energy band structure, density of states and optical properties of Mg2-xCuxSi(x=0.25) have been calculated systemically by first principle pseudopotential method based on density functional theory (DFT). The calculated results show that Mg2-xCuxSi(x=0.25) is a p-type semiconductor. The major contribution to the valence bands is made by the d states of Cu. The valence bands of Mg2-xCuxSi(x=0.25) are composed of Cu d and Si 3p. The static dielectric constant is 35.85, and the refractive index is 5.99. The electronic structure of Mg2Si can be modulated effectively by adding Cu atoms. The results offer theoretical basis for the design and application of optoelectronic materials of Mg2Si.