Abstract: To cope with the high NO_xemission problem of W-shaped radiant tubes,the influence of diluents in two-stage air combustion on the NO_xemission of W-shaped radiant tubes was investigated numerically. Calculation results coincide well with data obtained from experiments. The analysis was carried out for 3 cases:different diluents,stepwise dilution,and diluents together with high temperature air. Compared to diluting with N₂,diluting with CO₂ produces less NO_x. It can reduce the NO_xemission blow 4 ×10^-5 by increasing the diluted volume fraction under the condition of ensuring flame stabilization. By stepwise dilution,the secondary air contributes more to NO_xreduction than the primary air. The NO_xemission can be controlled under 1 × 10^-4 by increasing the diluted volume fraction,even if the temperature of preheated air reaches above 1000℃.