In some areas with rich water resources, hydropower station is a flexible power source which can be used to promote the renewable energy accommodation capacity of the power grid. This paper proposes a renewable energy accommodation capacity assessment method of regional power grid considering flexibility which can ensure full accommodation for renewable energy in the premise of safety and reliability. First, representative scenarios are generated using K-means clustering considering the correlation between the power sources and loads. Second, by optimizing the flexible generator dispatching, renewable energy can be fully used under normal operation states, and the expectation of load-shedding is minimized under fault conditions. Finally, with the data of a 110 kV regional power grid in a coastal area of South China, the relationship between renewable energy penetration and expectation of minimum load-shedding is captured, and the renewable energy accommodation capability of the selected network is evaluated by the proposed method, verifying the effectiveness of the method.