Abstract: Abstract: Considering the threat of ground obstacles in the process of spraying for the low altitude and low speed eppo unmanned helicopter, an obstacle avoidance method based on the improved artificial potential field was proposed. The ability of the helicopter to avoid obstacle was the key issue to improve the accuracy and the efficiency, which could be realized in the following steps. Firstly, because ground obstacles were various and of different shapes, most traditional artificial potential fields regarding obstacles as a mass point or a sphere was not conducive, and they could not spray precisely if ground obstacles were treated as the same type. To solve this problem, ground obstacles were divided into 2 types i.e. low and high ones, of which high obstacles contained both cylindrical and cubic obstacles. Their minimum safety areas were defined differently: cone was for low obstacles, cylinder for cylindrical obstacles and cuboid for cubic obstacles. To prevent the wide-angle maneuvers and improve the efficiency of spraying, 2 kinds of obstacle avoidance strategies were formulated, which were climbing over them for low obstacles and bypassing them for high obstacles respectively. Secondly, in order to conquer the defect that obstacles were unable to avoid the helicopter initiatively, an obstacle avoidance algorithm based on the improved artificial potential field was given by introducing the relative speed between the helicopter and the obstacle into the repulsive potential. Meanwhile, obstacle avoidance orders were given according to the proposed algorithm, such that the helicopter adjusted its velocity and altitude in real time. Then, in view of continuously changing payload weight during the spraying process, the flight controller was designed based on the adaptive back stepping theory, which was not only to track those obstacle avoidance orders precisely, but also to suppress the parameter uncertainty caused by the change of the payload weight. Both the obstacle avoidance algorithm and the flight controller constructed a whole obstacle avoidance system, aiming at ensuring the security of the helicopter. Moreover, how to combine the obstacle avoidance algorithm with the flight controller was clearly demonstrated. Finally, simulation results showed that the proposed obstacle avoidance method could avoid ground obstacles effectively and efficiently, whose obstacle avoidance path was shorter and obstacle avoidance time was less than the traditional artificial potential fields. For the low obstacles, the obstacle avoidance path was shortened by 66.7% and the obstacle avoidance time decreased by 31%; for the high obstacles, the obstacle avoidance path and time were nearly the same as the traditional artificial potential fields when avoiding cylindrical obstacles, but when avoiding cubic obstacles, the obstacle avoidance path was shorten by 42% and obstacle avoidance time decreased by 25%. On the other hand, the adaptive back stepping controller was verified with its effectiveness and stability. The helicopter could avoid obstacles quickly and smoothly whether its payload weight was 10 or 16 kg, even though the payload weight changed continuously in the process of spraying. In a word, the proposed method has a better effect, which can provide a reference for technical applications of eppo unmanned helicopter.