Using a phase Doppler-anemometer measurement system, the cold gas/particle-airflow behavior in a 1:40 scale-model furnace was assessed to study the influences of adjusting the inner-secondary-air ratio in a 600-MWe multi-injection and multistaging down-fired boiler. Numerical simulations were also conducted to verify the results of the modeling trials and to provide heat-state information. The results demonstrate that reducing the inner-secondary-air ratio from 19.66% to 7.66% gradually enhances the downward velocity decay of the gas/particle airflow, while the inner secondary-air downward-entraining effect on the fuel-rich flow is weakened. Lowering the inner-secondary-air ratio greatly inhibits the decay of the near burner-particle volume flux. In addition, the fuel rich-flow ignition distance is reduced, from 1.02 to 0.87 m. A lower inner-secondary-air ratio is harmful to restrain early NOx formation. Reducing the ratio also causes the fuel-rich flow to turn upwards ahead, while the penetration depth of this flow gradually decreases and the maximum temperature in the hopper region falls from 1900 to 1800 K. On the basis of these data, an optimal inner-secondary-air ratio of 13.66% is recommended.