Network vulnerability is not only associated with the topological structure of power grid, is also closely related to the electrical characteristics of the grid.In order to identify the weak link of power grid, based on the branch open circuit the trend of the entropy, this paper puts forward a new method for power grid vulnerability identification. This article defines the branch and node weak intensity index and established the weighted topology model based on impact entropy of power flow, thus it is concluded that the branch and node weighted index of betweenness, combined with weak strength and weighted index of betweenness, puts forward the comprehensive vulnerability index, which considers the influence of grid topology structure and electrical characteristics, and the introduction of the weighted betweenness can have the effect of amplification to the identification of grid vulnerability.Results can be found by an example,the comprehensive vulnerability index this paper defines can effectively overcome from the perspective of a single network vulnerability assessment, improve the accuracy and precision of the identification, illustrates the validity and rationality of vulnerability identification method.