Abstract: In order to study the effect of settling distance on droplet size distribution of plant protection UAV, T16 was used to conduct spraying experiment on tobacco. Based on the proportion of droplets number, proportion of droplets volume, and droplets coverage density under different settling distances, the mechanism of UAV droplets size distribution was revealed and optimization suggestions on UAV spraying operation parameters were put forward. The results showed that at a certain settling distance, the proportion of droplets number conformed to the Weibull distribution, and the proportion of droplets conformed to the Gaussian distribution. The smaller the settling distance, the smaller the droplets coverage density. However, the effect of settling distance on the proportion of droplets number and volume were both negligible. The proportion of 0～200 μm small-sized droplets number was 80%, while the proportion of their volume was 30%. The proportion of 200～400 μm medium-sized droplets number was 18%, while the proportion of their volume was65%. It was recommended to increase the nozzle flow rate and flying speed appropriately, as 1 L/min and 3.5 m/s were slightly low.Also the operating height should be set at about 2m.