In some areas of South China with rich water resources, hydropower station is a flexible power source which can be used to promote the renewable energy accommodation capacity of power grid. This paper proposes a renewable energy accommodation capacity assessment method of regional power grid considering flexibility. The method can ensure fully accommodating 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, a 110 kV regional power grid in a coastal area of South China is used to verify the effectiveness of the proposed method. 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.