The blackouts usually start from individual components failure which eventually leads to system crash, and a very few branches with heavy load play a promoting role in the tendency, therefore, it is very important to find these key branches, and analyze the system invulnerability under the conditions of branch failures. This paper builds the branch static energy function model and then proposes the branch state vulnerability factor. Combining with the structure vulnerability factor based on electric betweenness, the branch comprehensive vulnerability index is put forward. Through simulating calculation, the branch comprehensive vulnerability is evaluated to confirm the fragile link of power system. On this basis, the invulnerability index is determined. Through intentional attack and random attack, the influence of different disturbance on the network invulnerability is simulated. The simulation on the IEEE-30 bus system verifies the rationality and effectiveness of this method.