Identifying the key transmission sections of the power grid quickly, accurately and completely is of great significance for improving the stability of power grid operation and preventing the occurrence of blackouts. Firstly, taking into consideration the topological structure of the power grid and the distribution status of the branch power flow, redefining the weights of the circuits, establishing a weighted network model of the power grid to make it more in line with the complex and varied actual power grid;. Secondly, aiming at the limitations of the existing power transmission section search method, this paper proposes a method based on the improved Floyd algorithm to search the first k minimum weight path, which overcomes the leakage selection caused by searching only the line on the first shortest path. The progressive screening method quickly and completely identifies the key transmission sections, and can also effectively identify the lines whose power flow direction is reversed. The correctness and effectiveness of the proposed method are verified in the simulation results of the New England 39-node system.