To solve the current safety risk issues of UAV operations in low-altitude urban environments，a FBN was used to identify and analyze the key risk factors of low-altitude UAV operations. Firstly，risk factors were analyzed from the perspective of human-machine-environment-management based on operation process of low-altitude UAVs. GeNIe software was used to develop a Bayesian network(BN) for risk assessment of low-altitude UAV operations，and the prior probabilities of the underlying events were analyzed using expert prior knowledge and fuzzy sets. Finally，univariate，bivariate，and sensitivity analyses were performed，and the network feasibility was validated. The results indicated that the key risk factors for low-altitude UAV operation were UAV battery failure，environmental obstacles on the operation route，and UAV operation supervision technology. FBN reverse inference showed that environment-related risk (79%) and UAV equipment risk (60%) were the main risk factors in the UAV operation process.