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Industrial-scale experiments were conducted to study the effects of tertiary air declination angle(TDA)on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion(MIMSC)technology at medium and high loads.The experimental results indicated that as the TDA increased from 0° to 15°,the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced.The ignition distance of the fuel-rich coal/air flow decreased.In near-burner region,the concentration of O2 decreased while the concentrations of CO and NO increased.The concentration of NO decreased in near-tertiary-air region.The carbon in fly ash decreased significantly from 8.40％to 6.45％at a load of 260 MW.At a TDA of 15°,the ignition distances were the shortest(2.07 m and 1.73 m)at a load of 210 MW and 260 MW,respectively.The main and reheat steam temperatures were the highest(557.2℃and 559.4℃at a load of 210 MW,558.4℃and 560.3℃at a load of 260 MW).The carbon in fly ash was the lowest(4.83％)at a load of 210 MW.On changing the TDA from 15° to 25°,the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously.The change of TDA has little effect on NOx emissions(660-681 mg/m3 at 6％O2).In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic perfor-mance,an optimal TDA of 15° is recommended.