Primary air significantly impacts the ultra-low-load combustion of coal-fired utility boilers. In this study, eleven combustion scenarios with varied primary air ratios (the flowrate ratio of primary air to the total air, PAR) were numerically investigated for a 1050 MW double-tangential circular coal-fired boiler under ultra-low load. The effects of PAR on temperature distribution, coal burnout, and gas species were thoroughly examined. Meanwhile, special attention was paid to the reducing atmosphere near furnace walls for alleviating the problem of high-temperature corrosion. Results show that a relatively large high-temperature region in the furnace is achieved at PAR = 0.153, contributing to combustion stability under ultra-low load conditions. Although the residence time tends to be shortened as PAR increases from 0.123 to 0.275, the amount of unburned carbon in coal particles has a general trend of decreasing because of more available oxygen from primary air. Due to the high excess air coefficient at ultra-low load, strong reducing atmosphere and high-temperature regions mostly concentrate around the operating coal burners, posing corrosion hazard there. As PAR increases to 0.275, the area of high CO concentration regions near furnace walls and the maximum CO concentration decrease noticeably, indicating less pronounced high-temperature corrosion at large values of PAR. NO emissions increase from 294.4 mg/Nm³ to 823.7 mg/Nm³ with increasing PAR from 0.123 to 0.275. This is shown to be related to the weakened reducing atmosphere as well as the reduced residence time of the coal particles as PAR increases.