High-alkali coal contains relatively high contents of alkali metals, which can be usually released in the form of gaseous chlorides and hydroxides during combustion. The effect of alkali metals on NO formation is analyzed in an electrical heated drop-tube furnace at 800–1200 °C during coal combustion. Based on experiments and simulations, the mechanisms underlying the effects of Na salts on NO emission are clarified in CO/NH3/O2/H2O/Na additive (NaCl, Na2SO4, and NaAc) systems. The results indicate that the yield of NO initially increases and then decreases as the furnace temperature increased. As the temperature increased from 800 to 1000 °C, NO precursors (HCN and NH3) undergo accelerated oxidation to form NO. When the furnace temperature is greater than 1000 °C, due to the rapid precipitation of volatiles, a local reducing atmosphere is present around the pulverized coal particles, which inhibits NO formation. NaCl and NaAc addition significantly inhibit NO formation. However, the inhibitory effect is weakened at higher temperatures (>1000 °C). The Na2SO4 additive exerts little effect on NO generation during combustion because of its stable chemical properties. The same conclusion is also obtained from gaseous experiments showing that NaCl and NaAc significantly inhibit NH3 oxidation to form NO. Based on the results of calculations, NaCl and NaAc addition inhibits NO formation by promoting the recombination of H, O and OH and reducing the concentrations of radicals. According to the analysis of chemical reactions, the effect of NaCl and NaAc on NO formation is mainly determined by the competitive relationships among multiple reactions.