Pesticide application control is the key technology to improve operation quality, reduce chemical pollution, and cut down costs of production. A control system of variable pesticide application machine hauled by tractor was designed based on AT89C52 using hysteresis control method. The system can automatically change spraying volume according to the speed of the tractor in order to stabilize pesticide application volume. Experiments show that: when the acceleration of the tractor is less than 0.4 m/s2, the error between the actual application pesticide volume and the pesticide application volume set does not exceed 8%. The highly efficient system has good atomization effect. It not only can reduce pesticide residues and environmental pollution, but also can meet the needs of agricultural production.