Guided by the concept of “American First”，the Trump administration has been pursuing the “principled realism” strategy after coming into power. While paying much more attention to domestic affairs，the Trump administration takes a negative attitude toward multilateral cooperation and is unwilling to undertake international obligations. As a result，unilateralism is gaining momentum in its foreign policy. In order to achieve the goal of “Make American Great Again”，the Trump administration pursues trade protectionism and develops powerful military forces to preserve peace through strength. Returning the path of great power competition，the Trump administration regards both Russia and China as strategic rivals. In terms of specific policies，the Trump administration seeks extreme pressure on DPRK and Islamic state extremists while maintaining a high pressure on Iran. The Trump administration overhauled American strategy in Afghanistan and South Asia. While putting more pressure on Pakistan，it intends to lift India's role in the region. Regardless of the opposition，the Muslim ban made the relations with Muslim world worse.“American First” policies of the Trump administration have triggered a backlash from the international community. Consequently，the international image of the United States is damaged.Trump's self-righteous personality and the still lack of integration of his national security team will inevitably lead to a certain amount of vacillation in Trump administration's foreign policy.