Aircraft icing has uncertainty and randomness, which damages the dynamic performance of aircrafts and threatens flight safety. To solve this problem, this paper studies the envelope protection and control margin calculation of icy aircrafts. Firstly, the effects of icing on the aircraft safety envelope are analyzed by using the reachable set theory, which takes icing as the uncertain adversarial input. Then, based on the known flight safety envelope of the icy aircraft, two envelope border protection methods are proposed. The first is the improved optimal control border protection action, the control chattering phenomenon of the traditional optimal control protection method is eliminated. The second method is the safe border protection method that preserves the pilot's authority. By calculating the available control on the safety border, the pilot can be guided to complete the correct operation, so that the flight state does not exceed the safety envelope. Finally, the control margin of the current state point of the aircraft is calculated in real time to prevent the aircraft from exceeding the envelope in advance, and to guide the pilot to complete the operation with a high degree of safety. Moreover, a new iterative control margin calculation strategy is proposed, which can effectively reduce the conservatism of the control margin calculation.