The dust subjects to several forces when moving in the airflow. This paper divides the dust movement into horizontal/vertical directions, and studies the dust velocity variation rules in these two directions under the effect of gravity and fluid drag resistance force. Results show that in horizontal direction, under the fluid drag resistance force, during 0.0~0.2 s, the dust approaches to air velocity quickly, nearly the velocity of airflow, and the smaller the dust diameter is, the more quickly the dust is close to the air velocity. In the vertical direction, under the effect of gravity and resistance force, the dust has the tendency of sediment, and the bigger the dust diameter is, the bigger the final sediment velocity is. Each dust size has the corresponding final sediment velocity, and the final sediment velocity is 0.5 m/s for 5μm diameter of dust. The research results have provided theoretic basis for further study of dust transport behavior.