In order to ensure the application effect and avoid spray boom scratching crops in the work process, spray boom and plant canopy need to be maintained approximately parallel and proper distance. An online control system of spray boom height and balance was designed. The online control system used ultrasonic sensors to detect the height from ground to spray boom. The controller caculates the direction and speed of hydraulic cylinders by comparing the set spray boom height with the actual spray boom height which was detected by ultrasonic sensors. Then the controller sends out control signals of hydraulic cylinders to adjust spray boom height and balance. The structure of boom sprayer and the composition and working principle of spray boom height and balance hydraulic system were described. The software and hardware design of spray boom height and balance online control system were elaborated, and a spray boom height signal filtering algorithm of branches and leaves interference was designed and the online control method of spray boom height and balance was elaborated. The online control system of spray boom height and balance was tested by ground test and field test. Ground test showed that the system could effectively improve the spray boom height change caused by step excitation pavement. The spray boom height change could be controlled within ±3 cm. Field test showed that the spray boom height signal filtering procedure could effectively filter occasional interference of a small amount of cotton branches and leaves and realize online control of spray boom height and balance.