For the problem of protection misoperation caused by power system hidden failure. Proposing a new method about cascading failure risk assessment of power system which based on entropy TOPSIS. Established hidden failure model of overload protection on the basis of Sthe analysis about the cause of hidden failureS.Use the risk theory and risk function to quantify the four risk indicator which was over-load risk index、Loss of load risk index、voltage instability risk index and frequency deviation risk index to get risk index values. Then get the risk degree of each line based on the method of entropy TOPSIS. We can give the order of lines in the power system according to the results of the risk degree.Spoint out the fragile line in system. The simulation results of IEEE 39 node 10 machine system simulation results verify the feasibility and correctness of the method, provides the reference and basis to the maintenance and operation management of power systemS.