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In the past decades, many countries have suffered from serious blackouts which demonstrate catastrophic consequences caused by rare events in power systems. To address the emerged issues, the contingency screening and ranking should consider the risks of potential failures instead of the associated probabilities of following-up failures alone. However, it is computationally infeasible to evaluate all possible cascading failure sequences beforehand. Thus, fast risk assessment method should be developed to enhance computation efficiency while maintaining acceptable accuracy. In this paper, based on distribution factors and pre-contingency power flow conditions, a novel risk index is proposed to quickly assess the consequences of possible cascading failures. Then creditable cascading failure sequences can be determined and ranked according to the risk index. In addition, case studies on the IEEE-118 bus system show that the computational efficiency of the proposed cascading assessment method can be greatly enhanced with satisfactory accuracy, indicating a high potential of practical implementation.