In consideration of the current system operation state and important level of lines，a method of forecasting cascading failures to reflect variation of system operation state and system structure is proposed based on the operational risk theory and the multi-attribute decision theory. This method takes failure probability model of transmission lines under the factors of the external environment into account to calculate the possibility of line fault，which is combined with electrical betweenness index of lines and line association factor to build a comprehensive prediction index of cascading failure. Grey correlation theory adopting maximum deviation is used to distinguish the important level of lines structure. The simulation analysis on the IEEE 30 bus test system has figured out the cascading models which are serious relatively，and meanwhile proved out the validity and rationality of this method.