Abstract: Based on PWM (Pulse Width Modulation) technology, the variable pesticide spraying system of UAV (Unmanned Aerial Vehicle) was promoted, which consisted of kits, nozzles, pump and spray controller. The pump operating time was varied by the duty cycle of the PWM signal, so as to change the spray flow. According to the constant flow indicators of spraying pesticides and the coverage sprinkler system, the PWM frequency was determined to 1 Hz. A cubic polynomial relationship model between duty cycle and spray flow was established, and the coefficient of determination reached 0.9944. Experiments under flow rate indicator condition of 0.3, 0.4 and 0.5 L/min were done, and the results showed that the errors between sprayed and target flow were 8.66%, 4.50% and 1.60%. Variable spray control objective was achieved.