Abstract: The effect mechanism of Ca on nitric oxide (NO) heterogeneous reduction by char was investigated using density functional theory (DFT). The electronic structure of char model was analyzed to predict reactive sites. Mayer bond orders were used to quantify formation and breaking of chemical bonds in the reactions. There is a region with high electron localization function values in the extended outer region of unsaturated carbon atoms at the edge. The minimum electrostatic potential of char model, -101.1 kJ/mol, also exists at the edge, indicating the presence of lone pair electrons on edge carbon atoms. The doping of Ca could promote adsorption of the first NO molecule, but has little effect on that of the second NO molecule. The activation energy of rate-determining step is 124.4 kJ/mol for heterogeneous reduction of NO at the edge of char, whereas it is 91.9 kJ/mol at the Ca-decorated char edge. The kinetic analysis shows that the anterior factor increases after doping of Ca, meaning more sites are activated. The promotion of Ca to NO heterogeneous reduction is attributed to combination of the above two aspects.