The protection design criterion is proposed which is used in aero-engine structure static strength design and which requires that some sort of component failure which is less destructive should be occurred so as to prevent an aero engine from heavier damage. The necessity of protection criterion is discussed through practical examples and the definitions of the protecting part,protected part and protecting area are presented. Emphasis is put on the protecting part and protecting area used to prevent disk burst. Mathematical models and steps of protecting area design based on optimization are presented. Examples of protecting parts design of a driving shaft,a blade and a turbine side plate are presented,which theoretically and experimentally indicate that the proposed protect design criteria and method is efficient.