Abstract: Abstract: The major plant protection equipment in small fields is knapsack sprayer at present in China. When the canopies are tall or dense, the level/electric knapsack sprayers perform very poorly in droplet penetration. Motor knapsack mist sprayer is heavy, noisy and inconvenient to operate. An air-assisted electric knapsack sprayer was developed to improve the spraying efficiency of knapsack sprayer. The key component of the air-assisted electric knapsack sprayer is its atomizer with a motor installed behind the nozzle to drive a five-blade fan. When the motor rotates in a high speed to generate air flow, droplets are pushed to deposit to canopies. At the same time, the air flow blows and reverses leaves, so the droplets can deposit on both sides of leaves. The air-assisted sprayer can adjust its spray pressure from 0.15 to 0.6 MPa and the wind speed near air-flow outlet from 0 to 8 m/s. The atomization of 3 LECHLER hollow cone nozzles (TR80-01, TR80-02, and TR80-03) installed in the air-assisted electric knapsack sprayer was tested under the spraying pressure of 0.3 MPa. Droplet volume medium diameter (VMD) was measured by laser particle size analyzer, spraying angle and spraying sheet length were measured by high-speed photography, and spraying range was measured by water sensitive paper (wsp) under the wind speed of 0, 4, 6 and 8 m/s. ImageJ software was used to analyze the pictures taken by high-speed photography and DepositScan software was used to measure the density of droplets on water sensitive paper. Operating performance of air-assisted electric knapsack sprayer was measured in a tomato greenhouse compared with a conventional level knapsack sprayer. The tomato canopy was divided into 8 areas to determine the amount of pesticide deposition, 2 parts (left and right) from the vertical direction and 4 parts (top, middle upper, middle lower and bottom) from the horizontal direction. The distribution and deposition of the pesticides in tomato canopies were tested by LC-MS. The effect of biological control was also analyzed. The results showed that the droplets VMD increased with the increase of wind speed, and the spraying angle and spraying sheet length decreased with the increase of wind speed. Spraying range of the 3 nozzles (TR80-01, TR80-02, and TR80-03) under the wind speed of 8 m/s (the maximum wind) was 2.6, 2.0 and 1.9 times that without air flow. This air-assisted electric knapsack sprayer could increase the pesticides application efficiency and improve the pesticides distribution uniformity on the upper side and underside of leaves. The air-assisted electric knapsack sprayer using TR80-01 and TR80-02 nozzles could increase the application efficiency of pesticides by 1.38 and 1.14 times compared with the conventional level knapsack sprayer respectively. There was no significant difference in the control of Trialeurodes vaporariorum compared with the conventional level knapsack sprayer; there was also no difference for the air-assisted electric knapsack sprayer when its application dosage reduced by about 1/2 (TR80-01 nozzle) and 1/3 (TR80-02 nozzle) in the first 5 days after spraying, and the control result using air-assisted electric knapsack sprayer was better than that using the traditional level knapsack sprayer on the 7th day. The air-assisted electric knapsack sprayer is more quiet and lighter than engine-driving knapsack mist sprayer, and moreover it is suitable for spraying work in greenhouse because there is no exhaust gas. Different kinds of nozzles can be installed on the air-assisted electric knapsack sprayer according to the requirements of pesticide application.