In order to identify the vulnerable lines in power grid, the transfer entropy model is improved based on entropy theory. A branch vulnerability index based on weighted transfer entropy of power flow is proposed. This index takes line load rate into consideration to overcome the deficiency of previous models, which ignore the different properties of each branch, when measuring the distribution of the transfer power flow. A new branch integrative vulnerability assessment model is then proposed by combining with line electrical betweenness. This model makes a comprehensive evaluation to the vulnerability of lines from two perspectives: the dynamic process of interactions between lines and the Topology of distribution network. Through the simulation analysis of IEEE 39-bus system case, the proposed method can effectively overcome the perspective of a single network vulnerability assessment, verify the correctness and rationality of the proposed model.