In this paper a new approach about boundary study is developed by which the evolution of a kind of so called sensitive and reactive system can be dealt with. The new approach involves that the interactive relationship between a system and its environment leads to complexity. So a lot of important characteristics about system evolution collect to the boundary. The competition between the captive force and the repellent force in a system brings up a “bend boundary” which involves varied gradations. The factors on the“bend boundary”promote the system to restructure itself in a sensitive and reactive way．As a result the system can realize its emergence or coflapse and then complete one evolution．