In view of the complex low-altitude airspace environment and the fact that unmanned aerial vehicle（UAV） operation risks cannot be quantified， a low-altitude UAV operation risk assessment method considering the uncertainty in the operation process is proposed. According to the force model during the descent of the UAV， the area affected by the sudden failure of the UAV is deduced. Considering the uncertainty in the operation of the UAV， the Monte-Carlo method is used to determine the UAV’s fall point distribution， the impact area of the fall is calculated， and the system risk value is obtained. Through case analysis， the validity and rationality of the proposed risk assessment method are verified， which can provide real-time safety assessment for low-altitude UAV air traffic management， and guarantee the UAV operates safely in the low-altitude airspace.