Abstract: In order to realize variable-rate spraying of knapsack sprayer, frequency of 8 kHz square wave and adjustable duty ratio were evaluated to control micro electric diaphragm pump. By changing the on and off time of DC Motor, the application rates of knapsack sprayer based on the PWM could be controlled，and the spray characteristics were analyzed also. Results showed that the volume of spraying increased with increasing of the duty ratio, but the rate of volume decreased gradually, and the maximum and minimum volumes of spraying were 1.13 and 0.66 L/min respectively. With the increase of duty ratio and spraying volume, spraying area spread to both sides and the spray angle increased gently, and the lowest and the highest value were 41.53o and 61.78o respectively. The diameter of spray droplets were between 100 and 135 μm, and decreased with increasing of duty ratio. The droplets velocity increased with increasing of the duty ratio, and the velocity in the middle was higher than that in both sides obviously. The variable-rate spraying control system is stable which can be effectively applied to agricultural production.