The demands of aircraft quality design criterion on main control system failure and subsequently instantaneous response were analyzed.According to the simulation,the flight characteristics of an aircraft were studied in different angle of rudder seizure.It demonstrated that when rudder seizure with high angle and pilot could not take action immediately,the flight parameters would change sharply.The yaw angle increased 50 degrees in 5 minutes,side velocity could attain 40 meters per-second,the angle of attack and sideslip would surpass 30 degrees,roll rate would reach -20 degrees per second,side load would arrive 0.6g.Simultaneity the angle of attack exceeded the limited angle,the aircraft would stall.If control wasn’t working,the disaster would happen.These phenomena supply the sufficient information of the rudder malfunction. The validity of correcting yaw moment by asymmetry thrust was testified,the simulation results showed that even rudder seizure in most serious conditions,adopting asymmetry thrust can correct yaw moment caused by the rudder seizure.The judgment standards of flight safety level for the state of malfunction were given.The safety level was assessed caused by the rudder seizure.For an aircraft with two engines on one side,the pilots need to adjust the 4 engines to balance the asymmetric moment,the work load is increased enormously.According the flight safety standards,the safety level is level Ⅲ. The demands of aircraft quality design criterion on main control system failure and subsequent instantaneous response were analyzed. According to the simulation, the flight characteristics of an aircraft were studied in different angle of rudder seizure. At that when rudder seizure with high angle and pilot could not take action immediately, the flight parameters would change sharply.The flight angle would change sharply.The yaw angle increased 50 degrees in 5 minutes, side velocity could attain 40 meters per-second, the angle of attack and sideslip would surpass 30 degrees, roll rate would reach -20度 per second, side load would arrive 0.6g.Simultaneity the angle of attack exceeded the limited angle, the aircraft would stall.If control was not working, the disaster would happen. These phenomena supply the sufficient information of the rudder malfunction. The validity of correcting yaw moment by asymmetry thrust was testified, the simulation results showed that even rudder seizure in most serious conditions, adoptin g asymmetry thrust can correct yaw moment caused by the rudder seizure. the judgment standards of flight safety level for the state of malfunction were given. safety rating was assessed caused by the rudder seizure. For an aircraft with two engines on one side, the pilots need to adjust the 4 engines to balance the asymmetric moment, the work load is increased enormously. According to the flight safety standards, the safety level is level III.