Abstract: In agricultural aviation, quantitative acquisition of wind field（flow field） velocity distribution is important for single-rotor helicopter-assisted pollination operations and pesticide application efficiency improvement. As a complement to the wind field computational fluid dynamics simulation, field or laboratory measurement methods, this paper applies a semi-analytical theoretical method to rapidly and completely calculate the single-rotor flow field velocity. Based on the vortex ring model, the trailing vortex by the tip of the helicopter’s high-speed rotating propeller is equated to a vortex ring, and the wake vortices system under the propeller disk is equated to a cylindrical vortex surface formed by the continuous superposition of vortex rings. The wake shape is judged according to the motion state, and the vortex ring position and radius are determined by the fixed wake or predetermined wake, and the velocity of each point in helicopter rotor space is calculated analytically or semi-analytically by the vortex ring velocity induction equation. The comparison of the theoretical model with the computational fluid dynamics simulation and literature test data shows that the vortex ring model is simple and fast, and the calculation is completed in 2.8 s on an ordinary computer（CPU 2 GHz, memory 2 GB）. The average error in radial direction is less than 1.71 m/s, the average relative error is less than 39.1%, the average error in axial direction is less than 2.26 m/s, and the average relative error is less than 54.6%. When hovering, the induced velocity increases from the center to the tip of the rotor. In forward flight, the higher the leveling speed, the lower the induced velocity, and the lower the forward flight speed, the airflow outside the slipstream below the rotor changes from upwash to horizontal flow to downwash. The main factors affecting the flow field are forward flight speed, rotor radius, gross weight of the aircraft, and air density in that order. This model and results can provide a quick method and reference for quantitative calculation of rotor wind field to assist pollination operation and field application nozzle arrangement.