In this paper, based on the MIMSC (multi-injection and multi-stage combustion) technology, new burner arrangement and air distribution parameter settings were proposed for a 300-MW subcritical down-fired boiler originally using MBEL (Mitsui Babcock Energy Limited) combustion technology. Numerical simulations were conducted to study the influences of the secondary-tertiary air proportion on the airflow mixing effect in the furnace, the ignition and pulverized coal combustion characteristics. The airflow mixing effect in the furnace is characterized by the size of the dimensionless vertical velocity decay area (V-da) and the fluctuation of the maximum vertical velocity decay curve (V-dc). During the research, the sum of the secondary and tertiary air rate remained constant, and the secondary air rate was set to 30.54%, 33.54%, 36.54%, 39.54%, and 42.54%. It was found that under the condition of using new type burner, with the secondary air rate increased from 30.54% to 42.54%, the ignition distance of pulverized coal decreased from 1.30m to 0.84m, the dimensionless penetration depth decreased from 1.46 to 1.27, and the NOx emission and carbon in the fly ash decreased first and then increased. The variation of V-dc showed the following regulation. At the secondary air rate of 30.54% to 33.54%, increasing the secondary air rate decreased the fluctuation amplitude. While at the secondary air rate of 39.54% to 42.54%, increasing the secondary air rate increased the fluctuation amplitude. Only at the secondary air rate of 36.54%, V-dc in the tertiary air mixing area was smooth. When V-dc was smooth, V-da was small at 0.043, the carbon in fly ash at the furnace outlet was the lowest at 4.19%, and the NOx emission was low at 675.9 mg/m(3) at 6% O-2. An optimal secondary air rate of 36.54% is recommended. For the subsequent design of burner structure and parameters, it is suggested that V-dc in the tertiary air mixing area should be smooth. (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.