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In order to study the spray flow filed characteristic of large-scale plant protection unmanned aerial vehicle (UAV), based on the spray system of FR-200 large-scale plant protection UAV, the simulation platform of depositing spray droplets without plant canopy was established. The SST k-ω turbulence model and the DPM discrete phase model were adopted to simulate the deposition process of UAV spray droplets. By means of numerical Fluent simulation software, the flight speed, spray boom relative position and spraying angle impact on the spray flow field were studied,，field test was done to verify the results of simulation. The vertical velocity distribution of FR-200 UAV downwash flow field was unsymmetrical. The UAV speed had linear relationships with both the droplet swarm anti-drift coefficient and deposition, the total amount of the target droplets deposition density were 4.208μL/cm2 when flying speed was 3m/s, the total amount of the target droplets deposition density were 1.766μL/cm2 when flying speed was 5m/s. The anti-drift coefficient was 87.5%, 93.0% and 96.4% at sampling plane height of 0m, 0.5m and 1.0m when flying speed was 4m/s, respectively. With the increase of sample surface height, the anti-drift performance of droplets was promoted. The drift phenomenon of nozzles was serious which was installed on both sides of the spray boom. The nozzles were affected by the trailing vortex which caused the droplets drift. The rotor downwash flow increased spray droplets initial kinetic energy, the spray droplets acceleration was caused by gravity and downwash flow. The dispersion of number 5 nozzle spray droplets was increased due to the blocking effect, and it was hard to reach the sample surface due to the vertical kinetic energy attenuation of droplets. The smaller the spraying angle was, the stronger the droplets total anti-drift performance became.