Aiming at ballistic missile attacking, a two-sided optimal decision model used for ballistic missile attack - defense is put forward, asynchronous branch and bound algorithm is designed. Given a complete missile attack plan and a responding defense, expressing the enemy courses of action as a mathematical model which can cause maximum expected damage, and then the problem how to optimize our defensive interceptor pre-positioning to minimize the maximum achievable expected damage is shown and explained. A simulation instance that can show the validity of the two - sided optimal decision method used for ballistic missile attack-defense is given at last.