In order to obtain the deposition value of the droplets on the leaves of the fruit trees after spraying, a detecting system of droplet deposition on fruit leaves used in low altitude pesticide spraying was developed. The system consisted of dielectric leaf wetness sensor, data transmission module and host computer detection software. Through the calibration experiment of dielectric leaf wetness sensor, the regression equations of three solutions, which were tap water with 553μS/cm of EC, thiophonate-methyl with 860μS/cm of EC, and KH2PO4 foliar fertilizer with 1525μS/cm of EC, were established. The accuracies of the regression equations were verified by spectra photometry. Then, a wireless data transmission sensor system based on ZigBee was developed. Moreover, the Qt platform was used to write the host computer program including data analysis and display functions. Finally, the contrast test between water sensitive paper and this system was carried out by using WSZ-4X type plant protection UAV in a cherry orchard. The results showed that the coefficient of correlation between the density values of the droplet deposition obtained by two methods was 0.9626. For a single measurement point, the average error of the droplet deposition density was 22.8%. In the orchard experiments, under the influence of the wind speed and the unmanned aerial vehicle air flow and other environmental factors, the droplet distribution on the sensor and the water sensitive paper appeared certain differences. The consistency of the droplet deposition density obtained by two methods was preferable if the influence of environmental factors was ignored. However, the droplet deposition on leaves was acquired quickly, conveniently, timely and simply by using the developed system.