Variable rate spraying technology can improve the pesticide utilization ratio, reduce the waste of pesticide, and canopy volume was a very important variable rate reference in orchard spraying, so a variable rate spraying system based on canopy volume measurement was presented and developed. The system was mainly made up of PC, ultrasonic sensor, single chip, flow control mechanism and nozzles. Ultrasonic sensors were used to measure the distances between sensors and canopy foliage. Data detected by sensors were sent to computer by single chip for further processing. Flow control mechanisms were used to adjust the flow rates of nozzles real-timely. Hardware and software design were introduced. Hardware design included hardware choice, connections and installation positions designs, and software design included flow chart design and system operation interfaces designs which were used to display the measurement results real-timely and vividly, and some spraying controls of the system could be completed in the system operation interfaces.