According to the characteristics of tethered unmanned aerial vehicle (UAV) communication relay system, such as flexible deployment and long time staying in the air, the working principle of tethered UAV floating communication relay system is analyzed, the communication relay model is established, the main factors affecting the communication efficiency are analyzed, the combat application of UAV communication relay system is studied, and the future development trend is prospected. The results show that the research can provide a reference for the development of UAV communication relay technology system.