Graphitic carbon nitride (g-C3N4) can be used as a photocatalyst to reduce CO2. Doping is an efficient strategy for improving the photocatalytic activity and tuning the electronic structure of g-C3N4. The sulfur-doped g-C3N4 (S-doped g-C3N4) as a promising photocatalyst for CO2 reduction was investigated by density functional theory methods. The electronic and optical properties indicate that doping S enhances the catalytic performance of g-C3N4. From the reduction Gibbs free energies, the optimal path for CO2 reduction reaction to CH3OH production catalyzed by S-doped g-C3N4 is CO2 → COOH* → CO → HCO* → HCHO → CH3O* → CH3OH. In comparison with g-C3N4, doping S can alter the rate-determining step and reduce the Gibbs free energy from 1.43 to 1.15 eV. CO2 reduction activity of S-doped g-C3N4 is better than that of g-C3N4, which is in well agreement with the experimental results. Our work provides useful insights into designing nonmetal-doped g-C3N4 for photocatalytic CO2 reduction reactions.