In order to detect the fog droplet parameters related to the unmanned aerial vehicle (UAV) spray quality and its application in a variety of complex environments, a spray quality testing system for UAV based on waterborne acrylic resin and digital image processing technology was designed. The system consisted of droplet sampling module, image acquisition module, image processing module, wireless communication module, image and data storage module, and data display module. Among them, the water-based acrylic resin would have discoloration reaction when it met water, and it was made into a fog drop sampling module, and the fog drop image of the fog drop collection device was obtained, and then the digital image processing technology was used to process the fog drop image, and the fog drop parameters were calculated. The performance of the system was evaluated by UAV spray test in farmland environment. The system can detect two droplet sizes, droplet deposition density and droplet coverage rate in real time, so as to realize the detection of UAV spray quality. The preliminary experimental results showed that the system ran stably, and the distribution curve of detection results of the system was consistent with that of the water sensitive paper method on the whole. And it can timely and accurately detect the quality of UAV spray. The research results can provide data support for the further development of UAV automatic spray system and aviation drug application decision system.