The purpose of the research on safety boundary of multi-body separation is to find suitable safety design criteria or system analysis methods， so as to provide reference for relevant professional design and to provide optimization basis and solution for safety separation design. In this paper， the safety boundary of high Mach number forward separation of protective shield is studied. Criteria for safety boundary of typical forward separation between protective shield and projectile and an inequality for back-pursuing collision of protective shield are put forward. The judging criteria can be used for research on the safety of shields during the axial direction separation stage， and can also provide basis for the design of separation schemes and the selection of engine types. The back-pursuing collision inequality establishes the relationship between the motion parameters at the initial moment of free motion and the motion acceleration of the protective shield， which provides a reference for predicting the later motion of the protective shield. According to the initial state， the safety of the separation process of the protective shield can be judged by using the safety boundary criteria and the back-pursuing collision inequality， providing reference for the separation design.