Aiming at the problem of RTK-GNSS receiver’s high-cost and single-point GNSS receiver’s low data update rang, a GNSS/IMU-based method of measurement agricultural machinery speed is proposed, and a speed measurement device with ISOBus interface is developed to measure the speed, direction and distance of the of agricultural machinery travel. In addition, the measurement data in accordance with ISO11783 standard is sent to agricultural machinery ISOBus network, for precision agricultural intelligence equipment to provide real-time accurate speed data. By comparing with the RTK-GNSS receiver, the test results show that the GNSS/IMU-based speed measurement device can meet the requirement of precision measurement, distance measurement and direction of travel of the variable rate application under a normal operation condition of agricultural machinery.