Fly-By-Wire (FBW) system is safety-critical system for flight safety. Regarding safety evaluation problem of FBW system, flight risk small probability event evaluation method based on Extreme Value Theory (EVT) after flight control system's failure is proposed. Step of evaluating flight risk based on EVT is summarized. By contrasting the fitting effects under different coordinate transformations, the result shows that double negative exponential is the best method of approaching extreme value distribution of type. Taking flight safety evaluation after execution failure of one aircraft for example, probability of unsuccessfully eliminating the failure for the pilot and the integrated flight risk are calculated. According to flight risk in military standard, this aircraft can satisfy the safety requirements. Then an available method of evaluating safety of aircrafts' flight control system is proposed.