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Multi-rotor plant protection unmanned aerial vehicle (UAV) in sloping fields was usually operated at a height of 1.5~3m above the crops. The significance of the terrain following is that the height of the plant protection UAV above the crops do not change when the gradient of the terrain changes. The plant protection UAV needed to maintain a stable terrain following to ensure the safety of flight and spray uniformity. A terrain following method was proposed by using millimeter wave radar. The pre millimeter wave radar was used for slope judgment .When the slope was small, the height of the differential GPS and the height of the ground penetrating millimeter wave radar were filtered by Kalman filter, which can improve the accuracy. When the slope changed beyond the threshold,the height of the pre millimeter wave radar and ground penetrating millimeter wave radar were fused by multi-radar information fusion, which can improve the response speed. Then fuzzy PID algorithm was used to control the height of plant protection UAV. Through simulation and field flight test, the millimeter wave radar was accurate over the crops, and the height error of terrain following was less than 40cm. The results showed that the adaptability and the spray uniformity of the plant protection UAV in sloping fields were ensured. The research on terrain following laid a solid foundation for the automatic operation of the plant protection UAV under various terrains.