Since the Cold War ended in 1991, the international structure has transformed into a unipolar system dominated by US hegemony. Besides, the United States is a presidential system country. Therefore, the US president's decision-making on foreign policy has a major influence on the world situation. In particular, Trump has a strong personal style, which highlights strong influences on decision makers of US foreign policy. This article adopts historical research and literature analysis methods, trying to use four types of diplomatic ideology proposed by scholar Mead as a research approach, to analyze Trump's diplomatic thinking of making decisions. This article finds that Trump's diplomatic ideology has strong Jacksonianism and a little Jeffersonianism. Obviously it is inclined to isolationism and patriotism, with the sole consideration of US interests. Besides, Trump's diplomatic thinking also has major impacts on cross-strait relations at the political, economic, and military levels.