As a core standard of American media policy the public interest is a concept that has not been clearly defined yet. By tracing the history of American media policy and some key laws and clauses, this paper attempts to explore the nature of the public interest standard from three different stages: in the early media policies, the public interest standard fundamentally means constructing a commercial broadcasting system with orderly competition; In public trust model, the public interest standard mainly functions as the restriction of regulator’s power and trustee’s behaviors; But since 1960s, the market supremacy has dominated the FCC over media policy and the public interest standard is interpreted as consumer's sovereignty and economy benefits, which can be obtained by the market force. Generally speaking, the public interest standard of American media policy has two different representative schemes: the social value model based on the democratic theory, and the market model based on the economy theory. And the ambiguity of the concept is not only a kind of design, but also rooted in the distinctiveness of media’s double attribute.