A rotary-cutting device suitable for a four-rotor UAV was proposed to address the problems of missing tassels after ground detasseling machine and lacking of mechanized filling and detasseling equipment of female tassel detasseling for corn seed production. Based on the operation characteristics and stability influencing factors of the detasseling UAV, the force of the cutting tool when cutting the tassel and its influence on the anti-torsional torque of the UAV were analyzed, and the overall structure of the rotary-cutting device was designed. The rotary-cutting range of the rotary-cutting device was 44~150mm, the folding range was 541~1318mm, and the total mass was 4.03kg. Through the cutting modeling and simulation of the rotary-cutting part, the rotary-cutting method of the vertical feed cutting was selected, and the optimal parameter combination when the cutting anti-torsion torque of the rotary-cutting device was the smallest was obtained by the bench experiment. On the basis of the above, a field experiment was carried out by integrating the rotary-cutting device with the UAV. The results showed that the unit cutting force was 10.54MPa when the rotary-cutting speed of the rotary-cutting device was 3954r/min,the vertical feed speed was 5.9mm/s, and the cutting tool edge angle was 32°, and the rotary-cutting device could remove the tassel comprehensively. The damage rate of leaves for detasseling UAV in the field was 14.58%, and the fluctuation rate of height was 1.88%, which could meet the requirements of detasseling for corn seed production.