To further reduce unburned combustible in fly ash of the improved down-fired boiler with the eccentric-swirl secondary -air combustion technology, a breakthrough idea is proposed to significantly increase the primary air velocity by reducing the damper opening of vent air. In this study, the effects of the high primary air velocity on anthracite burnout and NOx formation are investigated via industrial experiments. With increasing primary air velocity, there is a significant change in the dominant role between the inhibition effect of pulverized coal concentration reduction and the promotion effect of the recirculation zone enlargement on the pulverized coal ignition. With increasing primary air velocity from 23.9 to 32.6 m/s, the flue gas temperature in the furnace hopper increases by approximately 75 degrees C. Besides, the increase of coal char burnout rate at the initial combustion stage is larger than that at the intense combustion stage, and the sum of NOx reduction rates at the initial and intense combustion stages varies from 57 to 61%. The boiler thermal efficiency basically rises linearly from 91 to 92.34% with increasing primary air velocity from 23.9 to 32.6 m/s, and the change of primary air velocity has little influence on the NOx emission at the furnace outlet. Compared with the original boiler, unburned combustible in fly ash decreases by approximately four percentage points, boiler thermal efficiency increases by 3.3%, and NOx emission decreases by 43.3% under the primary air velocity of 32.6 m/s for the improved boiler.