Large-scale power flow transferring caused by overload lines removal is an important reason for cascading overload trips. For obtaining transmission section consisting of the lines that affected largely by the power flow transferring, this paper proposes a fast search algorithm about transmission section based on path split between sources and flows. Firstly, the algorithm transforms the actual grid structure into a directed graph using graph theory, then establishes the sending end node-power transmission branch adjacent table and quickly generates path trees of sources and flows. Breaking distributed factors of the branches included in all the effective path chains between sources and flows are calculated. The lines that largely affected by the power flow transferring are identified. This algorithm needn’t search for entire network topology and doesn’t contain any large mass matrix operations, so it is efficient. Finally, the simulation results of IEEE 39 nodes prove the rapidity and correctness of the algorithm.