Abstract: Abstract: The ultra-low-volume precision spraying of agricultural UAV (unmanned aviation vehicle) is a key technology in the field of plant protection. There are still some problems in the application of the ultra-low-volume spraying of agricultural UAV in China, such as relatively large droplet diameter of hydraulic nozzle, complex centrifugal nozzle structure and high price. Based on the mechanism of swirl atomization and the modular approach, a new swirl nozzle is developed in this paper. The hydrodynamics and the atomization characteristics of the flow field in the nozzle are studied numerically and experimentally. The physical properties of the fluid and the effect of the atomizer geometry on the atomization characteristics are obtained. The results show that the breakup of conical liquid sheet occurs via unstable growth of sinuous waves on the surface of the liquid sheet. In the structural parameters of the swirl nozzle, the diameter of the nozzle orifice has a great effect on the atomizer performance. As the diameter of the nozzle orifice increases from 1 to 1.5 mm, the average flow rate of the spraying is increased by 46.23%. At the same time, the average of spray angle is increased by 29.77% and the SMD (Sauter mean diameter) is increased by 15%. In addition, the flow rate of the spraying is linearly proportional to the number of the swirl slots. The spray angle is also impacted by the inclination angle of the swirl slot. When the inclination angle is increased from 30° to 45°, the spray angle is reduced by 11.6%. The relative flow pressure on the nozzle inlet has a significant impact on the flow rate of the spraying and the diameter of the droplet. The flow rate of the spraying linearly increases with the relative flow pressure, while the Sauter mean diameter decreases nonlinearly with the relative flow pressure. The experiments indicate that the Sauter mean diameters of serial swirl nozzles decrease by 25%-35% with an increment in the relative flow pressure from 70 to 160 kPa. In order to achieve the variable spraying, the flow rate and the diameter of the droplet can be changed by using the PWM (pulse width modulation) controller to control the fluid pressure of the pump. The increment in flow viscosity will decrease the spray angle for the swirl nozzle. The Sauter mean diameter decreases monotonously with a proper increment of flow viscosity (less than 200% of the viscosity of pure water), which can produce higher atomization quality. The experiment shows that when the flow viscosity is 17.6% greater than the pure water and the pressure is 160 kPa, the swirl nozzle can produce the droplets with the Sauter mean diameter of 45.38 μm, which is 18.3% less than the case of pure water. The results from the present study have significant implications for the optimal design of ultra-low-volume atomizer in agricultural UAV. The present method and results on the spraying mechanism of swirl nozzle provide the important reference for researching and developing the ultra-low-volume variable spraying system.