Abstract: With the development of low-altitude technology, adopting a UAV to transport medical materials has become a key scenario for urban low-altitude applications.To control the urban low-altitude risk 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, the uncertainty of the low-altitude environment, and the danger of medical materials, three risk incidents, including conflict, collision and leakage, are proposed, and the corresponding risk factors as well as consequences are analyzed. Aiming to mitigate the total urban low-altitude risks, three risk control stages, including the conflict, collision, and risk avoidance in the medical drone transportation are proposed. Taking the safety interval as the key parameter, the multi-stage urban low-altitude risk assessment models are formulated. Introducing the Event model and Gaussian plume model, the safety interval assessment method for each risk stage is individually formulated for the medical drone transportation. Finally, the effectiveness of the model and method is verified by the case study and numerical analysis, where the computational results show that, the new model and method can assess the potential risks of cargo-carrying medical drone transportation effectively. They also can provide the safety interval values for the medical drone transportation in different situations. The safety intervals are affected by the UAV's operating pitch angle, operating speed, size, wind speed, altitude and the illusion amount of hazardous ingredient contained in the medical material. In the conflict avoidance stage, the safety interval in the longitudinal, lateral and vertical directions for the general medical drone transportation is 41.8 ~ 46.7 m、20.4 ~ 25.2 m和14.6 ~ 18 m, respectively. In the collision avoidance stage, the safety interval in the longitudinal, lateral, and vertical directions is 127.4 ~ 53.9 m、22.7 ~ 20.9 m和36.2 ~ 25.4 m, respectively. In the risk avoidance stage, the safety interval is recommended to 9.41 ~ 14.16 m.