Abstract: Ballistic missile defense (BMD) system is necessary to deal with the threat of ballistic missile. Firstly, the developments of BMD systems in U. S. and Russia are introduced. The operation flow of BMD system is described in a given battle scene.Then the technology status of BMD system is described from three aspects, i. e. , early warning system, intercept weapons, Command and Control, Battle Management, and Communications (C2BMC) system. Finally, the difficulties that the BMD systemfaces and the key research points for the future are summarized.