In order to improve the droplet distribution uniformity and avoid repeating and missing spraying, a spraying flow control method based on pulse width modulation (PWM) was designed, and based on the MCU of STM32F407VET6, a dynamic variable spraying system fused with multi-sensor was developed. Spraying system was controlled remotely and spraying parameters were detected and saved by ground measurement and control software which was developed by LabVIEW. The tests based on reality field environment were carried out under the condition of different flying speeds to test actual operation performance and spraying effect of dynamic variable spraying system of plant protection UAV. Operation performance was reflected by deviation between practical and theoretical flows. Practical flow of micro diaphragm pump was detected by a flow sensor and theoretical flow was calculated by a derivation formula. Spraying effect was characterized by droplet deposition, which was collected by water sensitive papers and polyester films at targeted flying speed of 1.0m/s, 2.0m/s, 3.0m/s, 4.0m/s and 5.0m/s. Rhodamine-B water solution with concentration of 0.1% was used as a substitute for pesticides in the tests. Concentration of Rhodamine-B was measured by fluorescence intensity protractor, droplet coverage density and distribution uniformity were identified through image processing. The results showed that when flying speed was changed from 0.8m/s to 5.8m/s, spraying flow was able to match the speed accurately based on the spraying system and control method, and average deviation between practical and theoretical flows was 1.9%. The optimum flying speed of spraying operation based on 3CD—15 plant protection UAV was 3.91~5.10m/s, during the variation range of speed, the effective sprinkling width was 5m, droplet coverage density was 18~41 per square centimeter, coefficient of variation was 34%~75%, droplet deposition was 42.1~52.4μg/cm2. The research result provided method for optimization of flying speed and also theoretical support to improve the quality of aviation operations.