This paper will describe the transformation of the security strategy between Taiwan and the U.S in to face Chinese reunification policy. China has been trying to bring Taiwan under its sovereign territory. However, in fact, Taiwan has been able to survive until today with democracy and its presidential system without direct interference from China. As a small country with limited territory, power, and human resources, Taiwan is trying to sustain life and develop its state sustainability by maintaining its cooperation with the United States, especially in the field of security cooperation. Although during the first proposal of reunification, in 1981 the cross-strait relations in a tranquil situation, traditional threats in the form of military invasion could happen in the future. It was true happening that Chinese sent ballistic missile in 1995 and Taiwan saw it as a threat. So, Taiwan and the U.S are not only putting cooperation in the arms trade, but realized the need for more strategic cooperation. Therefore, this paper attempts to explain the transformation of defense ties between Taiwan and the United States to face possible threats from China's reunification policy.