Abstract: Whether the component of power system is vulnerable or not not only depends on its position in network structure, but also is relative to the operation state. In this paper, the branches and nodes are defined as vulnerable sources by using complex network theory, and the improved electrical betweenness is applied to evaluate the structure vulnerability of power grid. Then, the influence of vulnerable source fault on power grid and its sensitivity to other vulnerable source fault are considered, and risk index is used to represent the vulnerability of operation state. As to the shortcomings of only considering the maximum operation mode, multi-load-level model is proposed to account for the possible differences of vulnerability under different load conditions, and the comprehensive vulnerability index can be calculated. The proposed method can make up the shortcomings of single-analysis structure or operation state, and efficiently identify the weak link which can cause blackout accident. Through the analysis and calculation of examples, the results show that the proposed method has strong recognition ability and high identification precision, which can provide a powerful reference for operation maintenance and planning.