Abstract: Abstract: Planting pattern of cotton can be optimized for the higher yield, quality and efficiency production in Xinjiang Uygur Autonomous Region, China. However, it is still unclear about the effects of machine-harvested cotton planting pattern on the agricultural machinery and techniques in the spray application of chemical harvest-aid. In this study, a field experiment was carried out to clarify the effects, with two planting patterns of machine-harvested cotton, including the "aimizao" planting pattern (six lines per film with the wide-narrow row spacing, 66 + 10 cm, R6), and the "kuanzaoyou" planting pattern (three lines per film with equal row spacing, 76 cm, R3), as well as with two chemical defoliant spraying machines, including an agricultural drone sprayer (UAV), and a ground-based boom sprayer (MTZ). A representative canopy of four individual plants was selected in each pattern-sprayer combination. Some parameters were also measured for each leaf, including the volume diameter, coverage rate, and deposition amount of defoliant spray droplets. A systematic analysis was made on the effects of planting mode on the deposition and dissipation characteristics of defoliant droplets. An optimal defoliation efficiency of the cotton before the plants were determined ready for the mechanical harvesting. The results showed that there were significant differences between the UAV and MTZ in the volume diameter distribution, droplet coverage rate, deposition amount, and loss magnitude of the defoliant spray droplets (P<0.01). There was the more uniform distribution in the volume diameter of the droplet, when the defoliant was sprayed using UAV. 2.60% proportion was found in the most effective droplet with the size between 100-300 μm for the MTZ, which was improved by 58.40 percentage points to 61.00% for the UAV. The droplet coverage rates of UAV and MTZ were 3.22% and 42.05%, respectively. The coverage rate of MTZ was about 13 times higher than that of UAV. But, the droplet deposition amount of UAV and MTZ was 0.49 and 0.69 μg/cm2, respectively. Among them, the deposition amount of MTZ was only 71.00% of UAV, indicating the low capacity but high concentration of the UAV. Furthermore, the coverage rate and deposition amount in the lower part of the cotton canopy decreased by 17.99%, and 17.63%, respectively, using MTZ, but decreased by 35.45% and 53.71%, respectively, using UAV, compared with the upper part of the cotton canopy. Therefore, the penetration of spray droplets in the UAV was insufficient to reach the lower canopy. The droplet deposition loss between the cotton canopy rows of the MTZ was 1.91 times as large as that of the UAV. Different planting patterns of machine-harvested cotton also showed a significant effect on the deposition of spray droplet. More importantly, the coverage rate and deposition amount in the lower canopy increased by 18.59 percentage points and 0.33 μg/cm2 (117.60%, and 125.60% for the relative differences), respectively, in the "kuanzaoyou" planting pattern, compared with the "aimizao". By contrast, there was the decrease in the coefficient of variation by 43.73, and 31.63 percentage points (43.83%, and 36.00% for the relative differences), respectively. It infers that the "kuanzaoyou" planting mode was improved the penetration and uniformity of droplets in the canopy, especially for the middle-lower canopy layers. There was a great benefit in the penetration of UAV. All of the final defoliation rates reached a range of 90%-94% after spraying once by the BGM or twice by the UAV, fully meeting the harsh requirement of cotton mechanical harvesting operation. In conclusion, the UAV chemical defoliant spraying at the planting mode of "kuanzaoyou" can be expected to reduce the potential impact on the cotton growth and soil environment, due to the non-contact operation and reasonable distribution of droplet size. The finding can provide a strong reference for the mechanical harvesting of cotton to the chemical defoliation.