In bulk power systems， cascading failures initiated by normal events often lead to blackouts. For the purpose of investigating how the failures spread in the power grids， finding out the key-lines affecting the cascading failures， this paper presents an approach to calculate the risk of line outage and identify key-lines causing blackouts based on the hidden failure model and risk theory. Taking IEEE 118-bus system for an example， this model considers line overload， hidden failure， control strategy and operational conditions. Simulation results indicate that improving a few key-lines identified at heavy loading can reduce the probability of blackouts， especially large-scale blackouts. They also show that improving a few key-lines identified at critical state can make the system out of critical state and avoid large blackouts.