Based on DC flow algorithm， the key correlation factors for cascading successive failures are analyzed. Furthermore， with the probability model of hidden failure of relay considered，the sensitive behaviors in cascading successive failures are analyzed, and effects of the key correlation factors on the sensitive behaviors are analyzed. Then，based on the key correlation factors and the sensitive indexes，and from the view of subjecting and actuating，the basic structure and algorithm for distinguishing sensitive areas in power network under the condition of cascading successive failures are proposed. Some examples with IEEE 39 bus test system show the rationality of the algorithm.