The electronic structure and optical properties of Mn-doped CrSi2 have been calculated using the first-principles pseudo-potential method based on density functional theory. The calculated results show that CrSi2 is an indirect semiconductor and the indirect band gap is 0.35 eV,the Fermi level enters conduction band and the band gap narrows after it was doped with Mn,with the indirect band gap width ΔEg=0.24 eV and CrSi2 changes into n-type semiconductor thereafter. There are some changes of optical parameters after doping. The static dielectric constant before doping is ε1(0)=32,then it changes to ε1(0)=58 after doping. Moreover the influence on electronic structure,density of states and optical properties of CrSi2 after Mn doping is analyzed as well. The work could provide theoretical basis for doping of CrSi2 materials in future research.