The grid vulnerability analysis is important to fault prevention and system operational security. By considering both the operational state and topological parameters， the grid vulnerability analyzing method based on the two-dimensional accumulation means is presented， in which the modified small world network model is employed under given power system conditions. The research focus is analysis of the mutual relationship of grid topological changes and operational state changes to estimate the impact of the power redistribution and topology changes on system vulnerability and to identify the vulnerable lines in the power system. The analyzing method is helpful to the monitoring and maintenance of some important equipments in a power system. The results of test examples have proved the correctness of the method proposed.