Since post WWII to now, America’s foreign aid experiences lots of alterations. But whoever be in power, foreign aid policy keeps an important status all along. Foreign aid is one of US’s most important foreign policies, which can help the US to actualize America’s national strategy and interests. The total sum of US foreign aid ranked the first place in the world, research on the US foreign aid policy can help us further understand the significance of foreign aid. In this article, I will introduce the concept of foreign assistance and its several patterns, and then combine with the United States’ foreign aid action to study the motivation of particular foreign aid policy. In the different condition, the US’s policy also showed multiple changes, so it is meaningful to analyze the underneath influencing factors. I will elaborate the elements effect on US policy making by means of theoretical analysis, this thesis will pay close attention to the debate between Realism, Liberalism and Constructivism. Diverse theories have different views on US’s foreign aid policy, from that we will learn how the ideology, interest and humanitarianism act on US foreign aid policy. Research on this question, we will know more about the function of culture and national identity, and the interaction between the identity and interest. Based on the issue’s complexity, qualitative analysis method, documentary survey and historical analysis method will be used to illuminate this issue, too. By detailed analysis, this thesis concludes as follows: although interest and humanitarianism II impact on US’s foreign aid, but the national identity and culture is the most important factor which act on US interest and then influence US foreign assistance.