Abstract: The risk analysis of reservoir group operation is always a hot and important topic, especially for short-term optimal operation considering flow time-lag. A short-term optimal operation risk analysis model considering flow time-lag is established according to risk identification-measurement and risk assessment and decision-making. Firstly, we choose the inflow forecast error and flow time-lag factor as the main risk factors, and then use the stochastic simulation method to estimate the main risk factors, thus get the risk rate under the determined operating strategy by coupling the two major risk factors as model input, finally apply the model to the short-term optimal operation of two hydropower stations on Yalong river. The results show that the model is not only simple and effective, but also has important significance to making power generation schedule or reservoir operation management.