The power grid security risk assessment is of great significance for preventing catastrophic failures. Due to the shortcomings of the existing security risk assessment methods in comparability and understandability of severity index, a security risk assessment method based on fuzzy cluster analysis is put forward. The method uses operational reliability theory, to build the grid failure model. based on the entropy theory, building system state entropy overcomes the shortage of traditional severity model which consider state homogeneity insufficiently. Refer to the comparability of index，using the fuzzy cluster method calculate global system severity index. Through analyzing the IEEE30 system simulation, it proves that the method can effectively identify N-1 and N-k contingency risks.