Objective To evaluate the effects of spraying defoliation and ripening agent on cotton before harvest using unmanned aerial vehicle (UAV).Method Four cotton cultivars including ‘Jinmian 18#’, ‘Beiquan 9#’, ‘Zhijin 13#’ and ‘Hexin 47#’ were sprayed defoliation and ripening agent with the spraying volume of 22.5 L·hm^-2 at 30 days before harvest by UAV. Aritificial spraying with an electric backpack sprayer was used as cantrol at the the spraying volume of 450 L·hm^-2. Droplet coverage rates in the canopies of four cotton cultivars were tested. The defoliation rates and boll opening rates were investigated at the 4th, 7th, 14th and 21st days after the application. The fiber qualities and yield components (boll weight and lint percentage) of four cotton cultivars were detected.Result The droplet coverage rates of four cotton cultivars showed a rule that coverage of the upper canopy was significantly higher than that of the middle and lower parts of the canopy. The higher the plant height was, the lower the droplet coverage rate of the middle and lower canopy was. Except for the boll opening rate of artificial spraying treatment on ‘Hexin 47#’, the defoliation rates and boll opening rates of other treatments met the requirements of machinary harvesting. The fiber quality and yield components (boll weight and lint percentage) of four cotton cultivars were not affected by the type of sprayers.Conclusion The UAV can be applied to spray defoliation and ripening agent before cotton harvest.