The integration of large-scale renewable energy brings great challenges to the flexible regulation ability of the power system. Current research on flexibility evaluation and optimal dispatching focuses on supply-demand matching and lacks consideration of the transmission capacity of the line. In this paper, considering the transmission capacity of the line, a novel flexibility evaluation and optimal dispatching method is proposed. First, the influence of the transmission capacity of the line on meeting the flexibility demand of a power system is analyzed. Considering the fluctuation and uncertainty of renewable energy and load, the bus flexibility demand and its allocation on the line are quantified. Then, an evaluation index of system flexibility deficiency is proposed from the perspective of resource flexibility margin and line flexibility transmission margin. Taking the evaluation index as the chance constraint, an optimal dispatching model considering the double margin of resources and lines is constructed. The optimization dispatching model is reduced to a mixed integer linear programming model by the linearization method and the chance constrained deterministic transformation method based on Latin hypercube sampling to reduce the difficulty of solution. Finally, a modified IEEE39-bus system and an actual power system are used to verify the effectiveness of the proposed method. The results show that the proposed method can reduce line congestion and load shedding, and improve the consumption of renewable energy.