In view of the rapid increase in the scale of wind power integration and the difficulty of accurately predicting wind power, an online transient stability risk assessment method considering the uncertainty of wind power output is proposed in this paper. Based on the transient stability participation factors, the dominant wind farms are extracted and merged; the weighted wind farm output Euclidean distance is used to cluster the scenarios; the maximum risk operating scenarios are determined for each type of scenario subset according to the impact of uncertain variables on safety and stability; scene screening is adopted for safety and stability assessment calculations. Through the above steps, a safe and stable operation risk considering the uncertainty of wind power output can be obtained. The proposed method helps dispatch operators to accurately grasp the safe and stable operating boundary of the system in real time. An example of an actual power grid verifies the effectiveness of the method.