The electronic structure and optical properties of intrinsic 6H-SiC, P substituted for Si, C doped and P interstitial doped 6H-SiC were calculated by the first-principles pseudopotential plane wave method based on density functional theory. The results indicate that intrinsic 6H-SiC is an indirect band gap semiconductor with a band gap of 2.052 eV. The band gaps of the P substituted for Si, C doped and P interstitial doped 6H-SiC decrease to 1.787 eV, 1.446 eV and 0.075 eV, respectively. The interstitial doping band gap decreases the most. P substitutional doped 6H-SiC causes the Fermi level guide band to move and insert into the conduction band, and the 6H-SiC becomes an n-type semiconductor. One level of P interstitial doping valence band crosses into Fermi level, so a P 3p impurity level appears in the gap band, and 6H-SiC turns into p-type semiconductor. Substitutional and interstitial doping increase the real part of dielectric function of 6H-SiC, while the imaginary part of dielectric function, absorption spectrum, reflection spectrum and photoconductivity redshift. Among them, P interstitial doped 6H-SiC has the best effect. The conductivity of the material is enhanced and the utilization rate of the material in the infrared band is obviously improved by P doping. The results provide effective theoretical basis for the application of 6H-SiC in infrared photoelectric performance.