Abstract: Abstract：With the development of agricultural aviation technologies and their application in agricultural production, plantprotection unmanned aerial vehicle (UAV) has been widely used to control pests and diseases of crops. The high speedrotation of the rotor in the UAV produces a powerful downwash affecting the distribution of pesticide droplets on the ground.Understanding spatial distribution of these droplets on the ground is important to evaluate application quality of the pesticidesand plays an important role in improving the spray system in UAV and optimizing its operating parameters. Current methodsfor measuring the droplet deposition distributions use a number of collectors placed regularly on the ground to receive thedroplets and measured their sizes; it is difficult for them to effectively obtain the deposition of all droplets resultdue to thedownwash of UAV. This paper presents a new method to resolve this problem by improving accuracy and spatial continuity ofpesticide droplets measurement applied by an unmanned helicopter. The flying parameters of a 3WQF-80-10 unmannedhelicopter used to spray pesticides were obtained from the high-precision Beidou navigation system, and the RQT-C-3fluorescent whitening tracer with mass fraction of 1.0% was used as the proxy for the pesticides. Two droplet collectionmethods: one used continuous strip paper and the other one used individual water sensitive paper, were used to measure thedroplets deposition distribution. We divided the experimental field into three areas, with Areas 1 and 2 spaced 3 m apart, andAreas 2 and 3 spaced 1m apart. A metal bracket 8 m log and 0.5 m away from the ground was placed in each area. Prior to theexperiment, a paper tape was fixed on the surface of the bracket and the water-sensitive paper cards were placed evenly in thearea 0.5 m away from the paper tape. There were one paper tape and 15 water sensitive papers in each area, and a total of sixspray tests were performed based on pro-designed flight parameters. The combinations of flight speed and flight height were:2 m/s and 3 m, 2 m/s and 6 m, 2 m/s and 9 m, 3 m/s and 3 m, 3 m/s and 6 m, and 4 m/s and 9 m. The paper tape was detectedby fluorescence spectroscopy analysis, and the water sensitive papers were scanned using an image processing software toobtain droplet deposition coverage rate. The results showed that distribution curves of the coverage rate obtained by the papertape method coupled with the fluorescence spectrum tracer were consistent with that obtained from the images of the watersensitive paper method, with the R2 being 0.88~0.96. Because not all fine droplets fell on the water sensitive papers due to theeffect of the high speed rotating rotor, the coverage rate curve measured by the continuous fluorescence method had multiplepeaks and the value of its coverage rate was higher than that measured from the water sensitive paper method. When theunmanned helicopter flew at speed of 2 m/s and height of 3 m, the coverage ratio obtained from the continuous fluorescencemethod was up 16.92% compared to that sampled from the individual water-sensitive paper method, while when the flightspeed was 4 m/s at height of 9 m, the coverage ratio in the latter was 97.77% higher than in the former. In terms of the impactsof unmanned helicopter operating conditions on coverage rate, when the helicopter flew at 2 m / s and height of 3 m, thecoverage rate of the droplets obtained from the two methods were the highest, being 8.34% for the continuous fluorescencemethod and 7.14% for the individual paper method. With the flight height and speed increasing, the spatial coverage rate ofthe droplets decreased. In summary, the high-speed rotor of UAV generates a downwash, making the droplets of pesticidesmove in different directions and resulting in a large spatial difference in their deposition on the ground. Therefore, thecontinuous sampling method is more adequate to evaluate the spatial distribution of the droplets. This study has implicationfor study on detecting deposition of pesticides and other agrochemicals applied by UAV.