Utilizing unmanned aerial vehicle (UAV) to carry 5G base stations to build emergency communication networks can flexibly provide stable and reliable wireless access in scenarios without basic communication facilities, such as military battlefields, disaster rescue and other emergencies. In this paper, we optimize the flight path of UAV airborne base station (ABBS) in 5G emergency communication networks. Firstly, we propose the comprehensive signal loss index (CSLI) for assessing the quality of communication service. CSLI is related to user priority, the free-space path loss and non-line-of-sight link in electromagnetic wave propagation. Then, particle swarm optimization was utilized to optimize flight path of UAV 5G ABBS through very few iterations. Simulation results show that the proposed method can significantly improve the communication service quality provided by UAV 5G ABBS.