One type of new automatic target detecting based on size of canopy with variable chemical dosage and air-flow of fan orchard sprayer was designed and developed to meet the demand of chemical pest control in orchards. Canopy parameter data scanned by infrared sensors and LIDAR (Light Detection and Ranging) were used to detect the target and to design spraying algorithm and PWM (Pulse Width Modulation) control system. Four integrated five-finger atomizers were equipped on each side of sprayer, independent brushless fan locate behind and close the five nozzles in one unit, each five-finger atomizer equipped with five nozzles and one independent brushless fan. The gasoline generator was adopted as energy source to realize fully automatic spraying and the independent brushless fan was instead of central fan without PTO (Power Take Off)-power to realize partly air volume according to canopy size. The field test showed that deposition volume of front and back of tree were 2.52uL/cm² and 1.67uL/cm², and the droplets coverage were 71.1% and 32.4% respectively, the minimum number of droplets was 46.2 /cm².