Small multi-rotor plant protection UAVs have problems such as pesticide drift and uneven droplet deposition. The numerical simulation of the two-phase flow field of coupled six-rotor UAV downwash airflow field and spray droplets was carried out by combining RANS equation, SST k-ω turbulence model and SIMPLE algorithm, and the effect of ambient wind speed on UAV downwash airflow and pesticide droplet deposition was explored. The accuracy of the numerical model was verified by the relative error within 15% between the spatial point wind speed test and the simulated value. The numerical simulation results showed that the incoming flow caused a vortex in the UAV downwash air field, and the impact of the incoming flow velocity on the flow field below the fuselage was greater than that on the flow field below the rotor. The stability of the downwash wind field was reduced due to large vortexes caused by the side wind. Symmetrical placement of nozzles under the rotor improved droplet deposition uniformity. The incoming flow caused droplet convolution, and droplet drift was increased as the incoming velocity increased. In combination with various factors, the UAV nozzles should be symmetrically arranged under the rotor blades. In the actual operation of six-rotor plant protection UAV, the UAV operation direction needed to be parallel to the outside wind direction in a clear environment, so as to reduce droplet drift and improve pesticide utilization.