The geometry, electronic structure and optical properties of Co doped CrSi2 were calculated and analyzed by the first principle pseudopotential plane wave method based on density functional theory. The results show that the crystal constants of Co doped CrSi2 have no obvious change and its band gap width increases. The impurity level appears near the Fermi level due to the influence of the 3d electrons of Co element. The imaginary part of complex dielectric function of the Co doped CrSi2 is observed a red shift in the low energy direction, and the intensity of optical transition increases in the energy range of less than 1.20 eV and more than 2.41 eV. The main peak of the absorption coefficient moves to the high energy direction, and the peak value increases, which improves the absorption of infrared photons of CrSi2 in the energy range of less than 1.38 eV and more than 3.30 eV. The main peak of photoconductivity moves in the high energy direction, and the photoconductivity increases in the energy range of less than 1.16 eV and more than 2.36 eV, which indicates that the doping of Co improves the photoelectric properties of CrSi2, especially in the infrared region. The calculation results provide a theoretical basis for the research and preparation of CrSi2 photoelectric device.