An optimal boundary protection method was proposed not only to ensure flight safety but also to maximize the performance of aircrafts. A critical parameter and its changing rate were defined as control objects. Based on the characteristic analysis of these two variables under optimal boundary protection, the adjusting amount and time were selected as two critical parameters to control the input signal. Accordingly, an analytical relationship between the optimal values of adjusting amount and time was developed. In order to avoid incidents and accidents caused by wing stall, the stall angle limit was estimated to obtain the dynamic flight boundary. In addition, an optimal boundary protection for wing stall under icing conditions was implemented through simulation. The results demonstrate that the proposed method is valid and effective. Therefore, it can provide an important novel approach for flight safety guarantee under icing conditions.