Abstract: A real-time trajectory planning method based on a three-dimensional risk field was proposed for unmanned aerial vehicle (UAV) trajectory planning in complex urban low-altitude environments. A three-dimensional risk model integrating static potential energy field and dynamic kinetic energy field was constructed, and a decision-making framework based on a four-level priority response mechanism was proposed. Through three-dimensional grid discretization and real-time risk calculation, continuous field intensity was converted into discrete control instructions, enabling dynamic perception and avoidance of multi-level threats. Simulation results show that the trajectory planning algorithm based on risk field could guide the target drone to fly safely in a dynamic environment: when the prediction time was 1.5 s, the average collision number was 0.1. This method provides effective technical support for autonomous flight of urban low-altitude UAVs.