According to the development of US Ballistic Missile Defense System （BMDS） and the operational concept of kill chain and kill net， the concept and their closure of the information chain， time chain， energy chain， recognition chain， precision chain within the kill chain of US BMDS are analysed with the typical example of U.S. ground-based midcaurse defense system against the North Korea’s intercontinental ballistic missile attacking American mainland. The kill net and its comprehensive defense capability of ballistic missile defense for American mainland and overseas theater are analysed with the typical examples.