Many blackouts at home and abroad show that the transmission section is the weak link in a large-scale interconnected power grid. It is helpful for the dispatching department to adjust operational mode in time to avoid large area blackout incidents by quickly searching the key transmission section and calculating the limit transmission power of the section. In this paper, a topological aggregation algorithm is proposed to search the key transmission sections of the power network. Adjacent nodes in the topology are aggregated to the main diagonal of the adjacency matrix by simple matrix transformation. The power flow distribution factor is used to identify the key transmission section by searching the network transmission section directly on the transformed adjacency matrix. The optimal model of the limit transmission power of the key transmission section is constructed using the krill herd optimization algorithm to accurately calculate the limit transmission power of the section. Work on the IEEE14 node system and IEEE39 node test system verify the effectiveness of the proposed method. This work is supported by the Key Project of National Natural Science Foundation of China (No. 51637004).