Abstract: With the development of low-altitude technology, adopting drones to transport medical materials has become a primary application scenario in urban low-altitude environment. To control urban low-altitude risks and ensure the safety of drone transportation, a method of assessing the safety interval of medical drone transportation is proposed. Considering the operational characteristics of drones, uncertainties in low-altitude environment, and the hazardous properties of medical materials, three types of incidents including conflict, collision and leakage, are identified. The corresponding risk factors as well as consequences are analyzed. Aiming to mitigate the total urban low-altitude risks, a multi-stage risk assessment model for medical drone transportation is proposed, including conflict, collision, and risk avoidance stages, with the safety interval as the key parameter. Introducing the Event model and Gaussian plume model, the safety interval assessment method at each risk stage is individually formulated. Finally, the effectiveness of the model and method is verified by a case study and numerical analysis. Results show that, the proposed model and method can assess the potential risks of medical drone transportation effectively. They also can provide the safety interval values in different situations. Furthermore, the safety intervals are significantly affected by key factors such as drone pitch angles, operating speed, sizes, wind speed, altitude, and hazardous substance leakage in medical materials. In the conflict avoidance stage, the safety intervals for general medical drone transportation are 41.8~46.7 meters longitudinally, 20.4~25.2 meters laterally, and 14.6~18 meters vertically. In the collision avoidance stage, the safety intervals are 127.4~53.9 meters longitudinally, 22.7~20.9 meters laterally, and 36.2~25.4 meters vertically. In the risk avoidance stage, the safety interval is recommended to 9.41~14.16 meters.