Dynamic storage capacity flexibility adjusting the reservoir storage capacity provides a new way to consume hydropower. However, the inflow of water from a reservoir is uncertain, and the use of dynamic storage capacity dynamically adjusting the upper limit of the water level during the flood season will lead to insufficient flood control storage capacity. In view of this, this paper proposes a distributionally robust economic dispatching method of power system considering the operational risks of dynamic hydropower capacity. It takes into account the uncertainty of runoff while considering the flexibility of dynamic hydropower capacity, and makes reasonable and safe use of the flexibility of cascade hydropower capacity to promote the absorption of renewable energy. Firstly, based on the historical data set of cascade reservoir runoff, the operation risk model representing the uncertain boundary of hydropower dynamic storage capacity is constructed, and then the two-stage distributionally robust model considering the operation risk of cascade hydropower dynamic storage capacity is proposed. The first stage is to develop the day-ahead scheduling scheme of the unit start-stop plan. The second stage is based on day-ahead dispatching plan by adjusting unit output to deal with the uncertainty of runoff. Then, the linearization technique was used to transform the nonlinear model into a linear problem and the column-and-constraint generation(CCG) algorithm was used to solve the problem. Finally, the numerical results on the IEEE-30 bus system verify the effectiveness of the proposed model.