The vulnerability, as the extension of safety problem in power system, has great significance to be further researched. In this paper, according to the calculation method of the electrical betweenness based on load transfer coefficient, considering the influence between the power infused of the generator and load nodes, the structural vulnerability assessment model and index is proposed, which reflects the network parameters and operation parameters in the current operating mode. Taking the actual output power as the weight of the load transfer coefficient, the assessment model, which considers the affect of reactive powers make assessment results more accurate. Finally, a chain of random attack and intentional attack are used to text on the IEEE39 node system, the paper verifies the effectiveness and superiority of the assessment method by the analyzing the changes of the biggest connected region.