Electrochemical reduction of CO2 on metal-free N-doped carbon (N-C) catalysts offers an emerging route to transform CO2 into value-added fuels. However, it suffers poor efficiency when operated at low overpotentials. Herein, we demonstrated that sulfur (S) addition could greatly enhance catalytic CO2 reduction activity and selectivity of N-C. The N,S-codoped carbon catalysts exhibited a high CO Faradaic efficiency of 92% and a CO current of 2.63 mA cm(-2) at a low overpotential of 490 mV, much better than that of the S-free N-C counterpart. By correlating dopant configurations with catalytic performance of model catalysts and combining theoretical density functional theory (DFT) calculations, we revealed that incorporating S in N-C leads to the generation of a higher density of pyridinic N that serves as the determining active site for CO2 activation. In addition, S dopants significantly promote the catalytic reactivity and selectivity of pyridinic and graphitic N by decreasing the free energy barrier for key intermediate *COOH formation. The experimental findings and theoretical understanding of the dual-doping promotion effects reported in this work have provided insights on designing high-performance carbon catalysts for CO2 conversion.