It is very important and necessary to ensure plant protection unmanned aerial vehicle (UAV) not to cross its working field border when it is performing spray job. Meanwhile, it is the key technology for farmers to keep their working UAVs safe and with high working efficiency. In order to guarantee the security and efficiency of working plant protection UAV, a highly efficient and reliable electronic fence system was designed and developed based on ray method, which could be used for detecting whether the UAV had crossed its working field’s border, and giving a warning in time when there was crossborder trend. The basic idea of the method was to generate a safety border inside the working field’s original border based on the area of field border and UAV’s flying speed. When the UAV was detected to fly inside the safety boundary, there was no need for crossborder detection; when the UAV was detected to fly between its working fields border and its safety border, it was needed to proceed detecting whether the UAV was going to cross the working field’s border, and a warning should be triggered by electronic fence system if the UAV was going to cross the border. The system would throw an exception when the UAV was out of its working field’s border. The system was tested and the results showed that the developed electronic fence system for plant protection UAV could be used to detect UAVs’ flying state in real time, and monitor whether plant protection UAVs crossed their working border and give a warning in time whenever there was any crossborder trend.