In view of difficult effective quantitative analysis on vulnerability of complicated power system after faults

the theory of power flow mediator is applied to the quantitative analysis of vulnerability evaluation of power system and its feasibility is quantitatively analyzed. Firstly

the topological structure of large power system is analyzed based on the complex network theory

and it is confirmed that large grid network has such characteristics as small-world networks and scale-free networks

that is

the topological structure of power network is fragile and has key vulnerable nodes. Secondly

based on the flow distribution of network at present operation status and also on the participation extent of node and branch in the network

the concept of flow number

in which the physical background is more complying with the background of power system

is introduced for scientific quantification of the important role of the node and branch in the power transmission of the network. On this basis

the structural vulnerability assessment index at partial fault stage of big network is constructed. Finally

the IEEE 30 node is simulated and analyzed so to verify its feasibility as the vulnerability assessment index of network. It is proved by the comparison with topological number index that the flow number can accesses the vulnerability of the network in the partial fault of during steady operation of the network.