Intruder detection and recognition are the important topics of UAV in sense and avoid technology. The key task is to detect whether there is an intruder in Optical airborne images obtained by airborne sensors during the flight of the UAV, and identify and locate the intruder. This is one of the key technologies for the safe integration of UAVs into the national airspace and for ensuring the safety of UAVs and manned aircraft. This paper focuses on the detection and recognition of intruder in sense and avoid technology, analyzes the difficulties in detection and recognition of intruder, summarizes the main methods of visual detection and recognition of intruder, and points out the existing problems in this field future development trends.