Abstract: With the improvement of the interconnection of power grid, resisting the cascading faults becomes a key problem. In a certain scenario, the key current-carrying branch accidents of power grid will lead to overload cascading faults, threatening the safe operation of the system, which is of great significance to the screening and identification of such critical lines. On the basis of improving the DC power flow model of reactive voltage, the dynamic evolution model of overload-type cascading faults is deduced and constructed. The model takes into account the adjusting effects of power balance and voltage stability devices and the corrective control measures of the system, and accurately simulates the dynamic evolution of the cascading faults of the interconnected system after branch failures, so as to analyze the influence of cascading faults on the system function and structure. On this basis, the importance evaluation index of current-carrying branches of power grid is proposed, and the importance index of each current-carrying branch is obtained through large-scale cascading fault simulation calculation, and finally the effective identification of key branches of power grid is realized. Taking the IEEE 39-bus system as an example, the effectiveness of the proposed model and method is verified.