The airflow velocity and airflow direction in the duct of rotary multi-layer sieve were simulated and analyzed by fluid dynamics analysis software FLUENT. By observing and analyzing the velocity cloud chart, it is found that the air velocity in the settling chamber was not stable and too high to settle the light impurities. It was found that the air velocity and air flow distribution in the chamber were affected directly by the dimension of the upper wind shield and the distance between the upper wind shield to the inlet of air. The air velocity in the settling chamber was reduced obviously after being optimized by enlarging the settling chamber, reducing the vortex, therefore improving the impurity sedimentation efficiency and reducing the energy consumption.