Air traffic self-separation technology of the next generation air traffic management system is a cutting-edge direction recognized by the civil aviation community， which transfers partial or total separation maintenance tasks from ground to aircraft. The air traffic self-separation is implemented by using the distributed mechanism to facilitate flexibility and efficiency of flights. This paper reviews and analyzes the state-of-the-art and challenges of the development in this field. Three aspects of the research progress and trend are highlighted， including airborne flight situation awareness， airborne trajectory planning and controlling， and air traffic autonomous operation and control. Three trends of the technology are summarized， and directions for future research are identified.