With the rapid construction of power grid, the emergence of long-distance, high-capacity transmission lines and the formation of the regional interconnected power grid have increased have increased the probability of power failure in large areas, this phenomenon has fully exposed the vulnerability of large-scale interconnected power system. In this paper, Based on the theory of electrical betweenness and voltage stability, a comprehensive vulnerability assessment index based on the improved AHP- Entropy method, which integrates subjective opinions of experts and objective data information about the node, is introduced for fully evaluating the vulnerability characteristics of power system and scientifically identifying potential weaknesses of the grid. Through the simulation of IEEE39 node system, it is verified the rationality and validity of the evaluation index, which can provide a reasonable and effective decision-making basis to guarantee the power system security defense.