The movement patterns of droplets on a UAV spray platform are quite different from those on a ground spray system because of its height from the target plants and the influence of the down-wash airflow. Few publications have mentioned how the UAV flying height influenced the spray width. Based on XV-2 UAV, by means of flow simulation and iteration calculation and experiments, the velocity characteristics of the rotor down-wash airflow and the movement patterns of droplets as influenced by the down-wash airflow were analyzed. The simulation results revealed that the velocity discrepancy from the middle to the outwards of the down-wash airflow led to an expanding tendency of airflow and thus an expansion of the spraying width. The spraying width was basically proportional to the flying height. The curling airflow around the rotor led to two peak values of droplet deposition within the spray width, which might also lead to an increase of spray drift. The experiment result showed that the effective spray width was 10m at a flying height of 6m and 12m at a flying height of 8m. The uniformities of deposition distribution were almost the same between each flying height. Considering the uniformities of deposition distribution, efficient spray deposition, spray drift caused by cross wind and injury of plants caused by down-wash airflow, the flying height of the UAV should be correctly chosen from a proper range based on its type and load. The experiment result was consistent with the simulation result. Therefore, the research result explained the influence of UAV rotor down-wash airflow on spray width and it can provide reference for the design of UAV spraying system and the choice of working parameters of UAV spraying.