During the spraying application of plant protection UAV, the rotor downwash will not only affect the droplet deposition, but also cause disturbance to the crop canopy. Revealing the characteristics of the canopy disturbing area is helpful to understand the application characteristics of the UAV, and can also provide a theoretical basis for optimizing the droplet deposition effect and the UAV spraying system. The effects of UAVs downwash on crop canopy were studied by the methods of aerial photography and machine vision technology. The results showed that there were significant differences in the characteristics of the crop canopy disturbing area caused by the single-rotor and multi-rotor UAVs downwash. Under a hovering situation the single-rotor UAV had a ring-shaped disturbing area with a large size, the multi-rotor UAV had a circular shape with a small size. Due to the influence of the asymmetrical structure of the UAV body, the disturbing area characteristics for the forward and backward movement attitudes of the single-rotor UAV were different. When advancing, this area was small and irregular, while moving backward it was large and distributed in a U-shape. Under the two flight modes, the multi-rotor UAV was basically the same in terms of size and shape of canopy disturbing area. Moreover, the flight speed had a significant effect on the characteristics of the canopy disturbing area. When UAV flying with a low speed, this area was more concentrated with a nearly circular distribution, and the plants were swung heavily;when operating with a high speed, the downwash was swept over crop canopy and the disturbing area was distributed in a strip shape, and the plants were swung lightly. As for the P20 plant protection UAV, when the flight speed was increased from 3m/s to 6m/s, the average lag distance of the canopy disturbing area was 1.77m, 2.71m, 3.61m and 4.31m, respectively. The lag distance of disturbing area was proportional to the flight speed of the UAV. In addition, the faster the speed was, the weaker the resistance of the downwash was to natural wind, which was not conducive to contain the droplets drift, and the position of the canopy disturbing area was unstable. Based on the research results, it was pointed out that plant protection UAV rotor downwash can only effectively promote the droplet penetration under the premise of the low operating speed and the overlap of the position for droplet deposition and canopy disturbing area.