The system safety analysis is widely used in the safety design of an aircraft. For lack of checks for dynamic characteristics of the whole system, the distribution of some reliability indexes may not be reasonable. A comprehensive verification of system dynamic characteristics can be realized through the safety-oriented risk assessment. This paper proposes a new method of distributing the reliability index for FTA (Fault Tree Analysis), based on the cut sets and the importance measure. With the safety design of the FBW (Fly-By-Wire) system for an aircraft's rolling channel as an example, the fallibility index of the top event is distributed. Then the distributed reliability indexes are modified, combined with the flight risk assessed by the Extreme Value Theory (EVT), which plays an important role in the system safety analysis.