Abstract: With the extensive application of plant protection UAV (unmanned aerial vehicle) in China, the research of its low-attitude and low-volume spray technology has gradually become a research hotspot. At present, the domestic research on the aerial spraying application of plant protection UAV mainly focuses on the effect of aerial spraying operation parameters on the distribution of droplet deposition, while neglecting the evaluation and test of the effective spraying width of aerial spraying by plant protection UAV. It is a prerequisite for precision spraying operation of agricultural aviation to evaluate the effective spraying width of plant protection UAV accurately, and it has great significance to the planning of operation route and the improvement of spraying quality. Therefore, the effective spraying width of single-rotor and multi-rotor plant protection UAV was evaluated with the evaluation method of droplet density and 50% effective deposition amount method which were commonly used in China by the tests of 12 times with different flight parameters; and different evaluation methods were deeply analyzed with the image processing principle of DepositScan, which is an image processing software used to analyze the results of droplet deposition with the image of water-sensitive paper and other cards. The test results showed that: The evaluation method of 50% effective deposition amount was more suitable than the evaluation method of droplet density for evaluating the effective spraying width of plant protection UAV 3WQF120-12, which had a relatively larger droplets size, and the volume median diameter of droplet was about 270-380 μm. The average effective spraying width of plant protection UAV 3WQF120-12 was greater than or equal to 4.44 m with the evaluation method of 50% effective deposition amount. The evaluation method of droplet density was more suitable than the evaluation method of 50% effective deposition amount for evaluating the effective spraying width of plant protection UAV P-20, which had a relatively smaller droplets size, and the volume median diameter of droplet was about 130-175 μm. The average effective spraying width was greater than or equal to 2.58 m with the evaluation method of droplet density. The evaluating results of the effective spraying width of plant protection UAV P-20 were in accordance with the actual situation. At the same time, the droplet deposition midline of plant protection UAV 3WQF120-12 had shift with the wind direction, as well as plant protection UAV P-20. In addition, the analysis showed that the relative error produced by the software of DepositScan was different with the spot image produced by the droplets of different size due to the limitation of current image processing techniques. The relative differences of average diameters between the software of DepositScan and the stereoscopic microscope were 34.1%, 16.3%, 7.8%, 1.4% and 1.2% for 50, 100, 250, 500 and 1 000 μm spots, respectively, and the relative measurement error of droplet diameter would increase with the droplet size decreasing. Therefore, we should choose the suitable evaluation method for the effective spraying width based on different droplets size of plant protection UAV. The evaluation method of 50% effective deposition amount should be chosen by plant protection UAV with a relatively larger droplets size, and the evaluation method of droplet density should be chosen by plant protection UAV with a relatively smaller droplets size. The results provide a guidance for selecting the more suitable evaluating method of effective spraying width for plant protection UAVs with different parameters, reduce the re-spraying rate and the missing spraying rate of aerial spraying operation, and improve the spraying quality of aerial spraying operation. The results provide the theoretical guidance and data support for precision spraying operation of plant protection UAV.