Abstract: To optimize plant protection unmanned aerial vehicle（UAV）operation parameters, mechanically harvested cotton boll-opening period was selected as trageted object．Using a DJI T50 UAV, nine treatments were designed with three flight altitudes（1.0, 1.2, 1.4 m relative cotton）and three flight speeds（4, 5, 6 m/s）．Droplet deposition characteristics on cotton canopy were quantitatively analyzed through water-sensitive paper sampling and ImageJ analysis．Results demonstrated that flight altitude and speed significantly affected droplet deposition（P<0.01）．When altitude increased from 1.0 m to 1.4 m, deposition on upper leaves surface decreased by 54.1%, while deposition on lower leaves surface reduced by 62.0%．As flight speed increased from 4 m/s to 6 m/s, deposition on upper leaves surface declined by 65.6%, while deposition on lower leaves surface decreased by 60.9%．Deposition on abaxial leaves surface was less than 10% of upper leaves and further diminished with parameters changes．Droplet distribution uniformity（CV value）increased by 1.1% to 13.5% with increasing flight altitude and speed．Recommended defoliation operation was combination of 1 m flight altitude and 4 m/s flight speed, where deposition on lower leaves（0.15 μL/cm²）met defoliation threshold and had the lowest CV value（31.9%）．