Plant protection UAV is characterized by high efficiency, safe operation and strong applicability, it has developed rapidly in recent years and has become one of the main prevention and control methods for pests and diseases. However, there still has some problems during the process of controlling cotton aphids, such as the droplet deposition law in the target spray area is not clear, the operating parameters are set unreasonably, and the droplet deposition characteristics and control effect for night operations are unknown. To solve the problems mentioned above, the experiments were carried out at the Korla Test Base of the Institute of Plant Protection of the Chinese Academy of Agricultural Sciences in 2018 and 2019. P20 plant protection UAV was adopted with different operating parameters (at flight speed of 3, 4 and 5 m/s, flight height of 1.5, 2 and 2.5 m, spray volume of 7.5, 11.3 and 15 L/hm2) to spray pesticides during the day and night, and the boom sprayer and spray gun were used as control devices. The Latin hypercube method was used in test design and each treatment was repeated 3 times. The test results showed that there was a significant difference in the number of droplets between day and night operations. Under the same operating parameters, the number of droplets deposited at night was on average 42.82% higher than that during the day, and the coverage rate of droplets was increased by 51.04%. At the same time, the penetration of droplets was better at night, and the number of droplets deposited in the middle and lower layers of cotton plant and the back side of the leaves were more than that in daytime. During night operations, the average number of droplets deposited in the middle and lower layers of cotton plant accounted for 34.79% and 22.07% of the total number of droplets in the vertical direction, and averaged 33.27% and 21.89% during the day, and that of the spray guns were 29.50% and 19.98%, boom sprayer were 43.30% and 15.84%. The average number of droplets deposited on the leaves back of the night operation accounted for 19.80% of the total number of droplets deposited on the front and back side of leaf, and 14.18% in the daytime, and the propotion of the night was 39.63%, which was higher than that of the day. The number of droplets deposited on the back side of leaf of each sampling layer of plant was upper layer> lower layer> middle layer. In general, the proportion of droplets deposited on the leaf back did not exceed 25% for UAV operation. Droplets deposited on the leaf back was less for spraying by spray gun and boom sprayer operation, which accounting for 7.09% and 0.20%, respectively. During cotton flowering and bud stage, in order to increase the number of droplets deposited and the penetrability of the droplets, it is recommended to set the UAV operating parameters to a flight height of 1.5 to 2 m, a flight speed of 3 to 4 m / s, and a large spraying volume, because the rotor wind field can effectively promote the penetration of the droplets only if the drone does not weaken the wind field and the droplets are not lost. In terms of the relationship between the reduction rate of cotton aphids and the number of droplet deposited on the back side of the leaves, they are positively correlated with each other on 1st day after application, but no correlation on 10th day because natural enemies. The results also showed UAV application at night is more effective to control cotton aphid compared with application in daytime, and a 20% reduction on the pesticide dose had no significant effect on the control effect of cotton aphid. This research can provide reference for the reasonable operating parameters setting of plant protection UAV, and also provide a scientific basis for the effective prevention and control of cotton aphids.