The consequences of power system losing stability by being subjected to large disturbance are very serious and even disastrous, so analyzing vulnerability of power system from the viewpoint of transient stability is extraordinarily valuable. This paper evaluates the vulnerability of power system from the viewpoint of transient stability safety quickly and quantitatively based on the transient energy function, combining the time simulation method and the direct method which are the two main methods of transient stability analysis, and at the same time considering the occurrence probability of contingency. It constructs correlative vulnerability index based on the probability of contingencies and transient energy margin, and revises transient kinetic energy based on the condition of critical cluster for improving the evaluation precision of vulnerability of power system under different contingencies and confirming the weak links of power system. Simulations of this method in IEEE 14-bus system demonstrate its rationality and effectiveness.