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In view of the uneven distribution of airflow inside the multi-air-duct sprayer, the air flow caused by the air outlet is disturbed and the droplet can not be evenly deposited on the fruit tree canopy. In this research, the length parameter of the inner baffle plate of the multi-duct sprayer was optimized. The Computational Fluid Dynamics (CFD) was used to simulate and analyze the internal airflow of the air supply system of the multi-duct sprayer based on Star-CCM+ software. The standard deviations of the wind speed of the wind outlet 1~6 at different guide plates were 0.7468, 0.6776, 1.4441, 5.1305, 4.5768 and 0.8209, respectively. Among them, the standard deviations of wind speed value at Point 1, Point 2 and Point 6 were less than 1, indicating that the change of deflector length has little impact on the speed change. The standard deviations of wind speed value at Point 3, Point 4 and Point 5 were large, indicating that with the change of deflector length, the wind speed at Air outlet 3, Air outlet 4, Air outlet 5 were greatly affected. On this basis, through the response surface analysis of Air outlet 3, Air outlet 4 and Air outlet 5, it was determined that, the length of Deflector 1 as 200 mm, the length of Deflector 2 as 60 mm and the length of Deflector 3 as 50 mm, was the optimal parameter combination. Under the optimal combination parameters, the wind speed values of symmetrical Air outlet 3 and Air outlet 6 were 39.135 and 41.320 m/s, respectively, with a relative deviations of 5.58%. The wind speed values of air outlet 4 and air outlet 5 were 33.022 and 34.328 m/s, respectively, with a relative deviation of 3.95%, which meeting the design requirements of sprayer. The indoor wind speed test results showed that the average wind speed of the upper layer was 15.75 m/s, the average wind speed of the middle layer was 20.83 m/s, and the average wind speed of the lower layer was 28.27 m/s, which met the end speed principle. The wind field was distributed according to the shape of the fruit tree canopy. The wind field of the left and right sides of the sprayer was symmetrical distributed and the air distribution was uniform. The work can provide a reference for the design of multi-duct sprayer.