The consequence of instability after power system subjected to large disturbance is very serious, even leads to catastrophic accidents. This paper uses multi-machine power system transient energy function based on the structure preserving models to get potential energy of each branch. Then combined the directed electric betweenness of complex network which fully considers the impact of generator output changes to branch power flow, branch state vulnerability and the importance of branch in the system are calculated. Finally a comprehensive vulnerability index is built to assess branch transient vulnerability, which considers the state and structure simultaneously. Simulation results on the 6 machine system prove that this method can identify the vulnerable lines and the critical machines effectively.