Energy band structure and optical properties of the cubic Ca2P0.25Si0.75 with strain on the (111) surface are calculated by the First Principle pseudo-potential method based on the density functional theory (DFT), and the effect of strain on energy band structure and optical properties is analyzed. The results show that under the compressive strain range of - 8% ~0% , it is direct semiconductor but the band gap decreases with the increase of strain, the conduction band moves to low energy while the valence band moves to high energy; when the tensile strain is 0%~2%, the band gap increases with the increase of strain, when the tensile strain is 2%, the direct band gap is maximum, Eg=0.60441 eV; when the tensile strain is 4% , it turns to indirect semiconductor. The dielectric constant and the refractive index of cubic Ca2P0.25Si0.75 increase with the tensile strain; when the compressive strain is - 2%~0% , the dielectric function and the refractive index decrease; when the compressive strain reaches -2%, the dielectric function and the refractive index are minimum, then they increase with growth of the compressive strain. The absorption and reflectivity decrease with the increase of compressive strain, while they increase with the increase of tensile strain. The energy band structure and optical properties the cubic Ca2P0.25Si0.75 are influenced by strain, which is an effective means to adjust photoelectric transmission performance of cubic Ca2P0.25Si0.75.