An experimental study of the SNCR process with urea as reducing agent and sodium salts as additive has been carried out, and detailed analysis of the reaction mechanism has been given here. In the temperature range of 800-975 ℃, NO concentration decreases at first and then increases while the concentration of N2O increases at first and then decreases with the increasing of temperature, and the turning point is 900 ℃. With increasing of normalized stoichiometric ratio of reduction nitrogen to NOx (NSR), NO removal efficiency increases, while the concentration of N2O also increases, which decreases overall NOx removal efficiency. With sodium salts as additive, the concentration of N2O decreases with increasing of sodium salts addition at all temperatures, while the concentration of NO decreases at first and then increases at low-temperature side of the temperature window and increases at high-temperature side with additional increasing, whose changing extent is smaller than N2O. Since sodium salts as additive can remove N2O effectively and have no large influence on the removal of NO, the effect of sodium salts as additive is the combined effect of the production of active radicals and the removal of HNCO produced by the decomposition of urea through neutralization reactions, which is more important. To achieve the same effect under each condition, the needed addition of NaOH and CH3COONa is less than that of Na2CO3 counting as Na atom. For the decomposition of CH3COONa can produce CH3COO, its addition can promote the reduction of NO more obviously at the lower temperature than Na2CO3 or NaOH. Overall NOx removal efficiency at 900 ℃ with NSR=1.5 had been improved from about 30% to 70.45% through the addition of sodium salts. Sodium salts as additive caused the flue gas to become alkaline gas, but it was not serious for sodium salts existing as NaNCO.